We find the Hebrew root “צ.נ.ע” twice in the Bible; once in Micha (6:8) and once in Proverbs (11:2):
1. הִגִּיד לְךָ אָדָם מַה טּוֹב; וּמָה ה' דּוֹרֵשׁ מִמְּךָ, כִּי אִם-עֲשׂוֹת מִשְׁפָּט וְאַהֲבַת חֶסֶד וְהַצְנֵעַ לֶכֶת עִם אֱלֹהֶיךָ
2. בָּא-זָדוֹן וַיָּבֹא קָלוֹן; וְאֶת צְנוּעִים חָכְמָה
In both places it means to be secretive about ones good deeds, and not to be overly flamboyant. That meaning has lasted through to the Talmudic usage - much as the meaning of "modest" in English connotes the opposite of “proud”.
There has been somewhat of a change in the usage of the word in Midrashic, and later in Talmudic literature to mean modesty in an erotic sense. For example אלעזר בר צדוק is quoted as saying (in Sukkah 49b) that this verse (1.) explains three of the most important facets of Judaism, and in regards to “הצנע לכת עם אל-היך” says this part of the verse is really discussing the need to accompany the dead, and accompany a bride. He adds though “if the Torah tells us to act modestly for things which people usually do publicly, how much more so should we be ‘modest’ in regards to things that are done in private”.
It’s also important to remember though, that 1. Halakhah as we know it is somewhat of a "created" thing, being that the Halakhic codifiers had to sift through the entirety of the Tanach and Talmud and piece together little statements, and many times just inferences, and many times what we called in my yeshiva “unnecessary inferences” (inferences that aren’t the only possible thing that can be inferred). And 2. There’s no ‘מצווה’ of 'צניעות'. It’s something that’s (like I said about hair-covering for a woman) expected of women, and especially of a Jewish woman.
In Iraq, in the era in which the Talmud was written, there was honestly not much of a problem in regards to tzniut; the norms there were obviously much more "conservative" then they are now. So all you’re going to find in the Talmud are statements like “one should not pass by when women are washing clothes in the lake” or “one should not watch a woman weaving” etc. Most of what we 'know’ about the halakhot of tzniut come from statements like that.
2. Modesty in Dress
First of all, some parts of the body are usually covered anyway, so the main concerns among the Orthodox Jews are the covering of the upper arms, legs, and neckline (an issue less necessary of concern among Orthodox Jews are the back, cleavage and belly).
In regard
s to wearing a skirt instead of pants: 1. Some see it as a transgression of "No male article shall be on a woman, and a man shall not wear a woman's garment", 2. The Talmud says it's better for a woman not to ride a horse (seemingly because it will cause the shape of her legs to become more evident when separated), so that could also translate to not wearing tight pants. Though in that case a tight skirt/dress is just as bad, and either way; it's unusual for a woman not to wear pants in most parts today. Either way; women who wear pants should obviously not be treated as if they're not part of the Jewish (or Orthodox) Community.
[I think the clearest example of learned and pious Jews who are somewhat lax in
these areas (where the husband doesn't wear a head covering and the wife doesn't have a hair covering and wears pants) is Sefaradim in South American countries; I've met a lot of them, and many of them are just as 'pious' as any New York Kolel couple.. ]
About the covering of the upper legs and arms: Like I said, one of the major logical sources is that the Talmud says that washing clothing and weaving (by a woman) are seen as things which should be done in as private a setting as possible, being that parts of the upper arms and legs might become exposed in the process.
About head covering: There has always been somewhat of a difference of opinion about "פריעת הראש"; does it mean ‘untying’ or ‘uncovering’. For example it says "even an unmarried girl shouldn’t have her head ‘פרוע’ "; some strict opinions say this means even an unmarried girl should cover her hair, and some say it means her hair should usually be tied. (See Rambam source).
It’s important to note that according to these standards there’s nothing better about wearing a blouse and long skirt to a jeans skirt and tee-shirt (for example). There’s no source, therefore, for Orthodox girls to just dress really "fancy" or however one would term current ultra-orthodox dress. If anything there have been contemporary national-religious rabbis in Israel who say that sometimes “
too much tzniut can lead to a lack of tzniut", something I see as very true. Not to mention concentrating on such things too much (instead of Torah and mitzvot) can lead to too great a concern for sexually oriented matters.
It should be mentioned that men must also practice modesty in clothing, and women must also not ‘gaze upon immodest men’, etc..
3. Other InterpretationsIt should also be mentioned though that some (Reform and some Conservative Rabbinical authorities) see these Talmudic standards as having only been applicable to Jewish women when that is what the standards of the times were, but now that the standards have changed the Talmud would agree there’s nothing wrong with a Jewess in a short sleeveless dress when the weather makes it necessary, and such a thing is not seen by anyone as breaking any barriers (though the growth of the Muslim community as well as a new stress on modesty in dress among the Evangelical community could still change those standards again). (See Ritva source).
4. True Modesty
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According to all opinions though a real problem has developed in 'our' communities, which, in a way exists among females in the Ultra-Orthodox community just as much as it does with the males, and that is; "depending on your clothing to
be religious for you". It’s not just about "women dressing modestly", it’s about "everyone
being modest", not just in dress, but in action; in all areas of life. Women should obviously be a little more modest than men, but it is applicable to both sexes obviously).
Much of what "צניעות" should (still) be about is doing all actions in a refined, restrained, chaste and modest manner- even things like speaking and putting on ones shoes, and definitely things like how one walks, laughs or bends over (when necessary) in a public place (among a myriad of other examples).
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Some practical examples of this type of behavior:
It says in Genesis; "And they said unto him, Where is Sarah thy wife? And he said, Behold, in the tent", which shows Sarah didn’t go out for no reason. Similarly the pasuk in Psalms 45:14 "כל כבודה בת מלך פנימה, ממשבצות זהב לבושה" ("The king's daughter is all glorious within: her clothing is of wrought gold") is used to suggest that a woman shouldn’t go out much. Though there's also the issue with the "אשת חיל" in משלי who seems to be very outdoorsy, so..it's complicated...still needs looking into...
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Also, it says that רות (and רבקה) bent in a restrained manner to pick things up.
Another example is is ישעיהו's rebuking Judean women for walking in 'too arousing' a manner (Isaiah 3): "Moreover the LOR"D saith, because the daughters of Zion are haughty, and walk with stretched forth necks and wanton eyes, walking and mincing as they go, and making a tinkling with their feet; therefore the LOR"D will smite with a scab the crown of the head of the daughters of Zion, and the LOR"D will discover their secret parts. In that day the Lord will take away the bravery of their tinkling ornaments about their feet, and their cauls, and their round tires like the moon, the chains, and the bracelets, and the mufflers, the bonnets, and the ornaments of the legs, and the headbands, and the tablets, and the earrings, the rings, and nose jewels, the changeable suits of apparel, and the mantles, and the wimples, and the crisping pins,the glasses, and the fine linen, and the hoods, and the vails. and it shall come to pass, that instead of sweet smell there shall be stink; and instead of a girdle a rent; and instead of well set hair baldness; and instead of a stomacher a girding of sackcloth; and burning instead of beauty." (Some even use the examples given in this verse to represent what shouldn't be worn by a woman).
It also says in the Talmud, for example, that a Roman soldier once took interest in a Judean girl because of her elegant manner of walking (so we see that one should refrain from such a mannerism).
To conclude, I should mention that רש"י on the first page of מסכת סוטה explains the גמרא as saying “The modest woman ends up with the ‘righteous’ husband, while the immodest woman ends up with the ‘wicked’ man as a husband”.
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5. Rabbinic Sources