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What is the source for the prohibition against Nivul Peh (using foul language)? Is it Biblical? Rabbinic? "Asmachta" (based on a principle derived from the Torah but ultimately not considered a Biblical mandate, ie., not a Mitzvah)? And based on the answer to the above, how severe is the prohibition (when can it be overridden, what, if anything, is the penalty, etc.)?

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Note that this is the first time I've used Lomdus (that I remember), and that I'm trying to use it in accordance with DoubleAA's scheme. – Seth J Aug 1 '12 at 17:24
Is my 'scheme' policy? – Double AA Aug 1 '12 at 17:31
related/answer judaism.stackexchange.com/a/14260/759 – Double AA Aug 1 '12 at 17:35
@DoubleAA, dunno. I was just pointing it out in case anyone saw it differently or questioned why I was tagging both lomdus and halachah-theory – Seth J Aug 1 '12 at 18:34
2 add. sources: gemara pesachim about לשון נקייה, & Rambam's explanation of לשון הקודש – wfb Oct 25 '13 at 19:35
up vote 4 down vote accepted

Isaiah 9:17-

Therefore the Lord will take no pleasure in the young men, nor will he pity the fatherless and widows, for everyone is ungodly and wicked, every mouth speaks vileness. And for all this, his anger is not turned away, his hand is still upraised.

As @GershonGold pointed out, the Talmud (Shabbos 33a) derives from here that nivul peh is a very bad thing. It would seem strange that a Prophet would be decrying a certain behavior, especially in such terms, if it only went against a decree of the rabbinic courts, and not something defined as wrong by God himself. If we can't find an allusion to the wrongfulness of nivul peh in the Torah (i.e. the Pentateuch) then we should look harder. From textual context alone I don't think it makes sense to equate this with lighting Chanuka candles or muktza.

One suggestion: The Rambam understands the concept of refining ones character to be included in והלכת בדרכיו - the obligation to walk in the ways of God. I'd assume this could fall under that.

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Kedoshim TiHyu? – Seth J Aug 1 '12 at 19:30
@SethJ Perhaps. – Dov F Aug 1 '12 at 19:32
@SethJ Note that Rashi's pshat on that is to keep far away from arayos; naval birshus hatorah is Ramban – b a Aug 2 '12 at 21:32
@SethJ Added a possible suggestion.^^ – Dov F Aug 2 '12 at 21:43
@Dov F,.. Nice. – Seth J Aug 2 '12 at 21:45

Yerushalmi Terumoth (ch. 1):

ולא יראה בך ערות דבר (דברים כג): ערות דיבור זה - ניבול - פה

Also, see Mesilat Yesharim (ch. 11) about the severity of this sin, where he brings this and other sources.

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שבת לג ע"א says that due to the sin of Nivul Peh many Tzaros and bad Gezeiros happen and young people pass away. So I guess it is Rabbinic in nature.

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