The Collaborative International Dictionary of English v.0.48:

faux pas \faux` pas"\ (f[=o]` p[.a]"). [F. See False, and
   Pas.]
   A false step; a mistake or wrong measure; especially, a slip
   or misstep in social etiquette; a blunder in conduct or
   manners that causes embarassment; as, he committed the faux
   pas of mentioning John's former wife.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

faux pas
    n 1: a socially awkward or tactless act [syn: faux pas,
         gaffe, solecism, slip, gaucherie]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "faux pas":
   blooper, blunder, boner, boo-boo, botch, break, bungle, gaffe,
   howler, impropriety, indecorum, indiscretion, misjudgment, misstep,
   oversight, pratfall, screamer, solecism, stumble, stupidity