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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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.
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.
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.