A faux pas is a social blunder — an act or remark that breaches etiquette and embarrasses the person who commits it. English borrowed the phrase whole from French, where it means literally "false step."
The term names a violation of manners rather than of morals, and the distinction is what makes it useful. Using the wrong fork, congratulating a woman who is not pregnant, asking a dinner guest what they earn: the damage is to standing and comfort, not to anyone's rights, and the offense is nearly always unintentional. Deliberate rudeness is an insult, not a faux pas. Because the standard being violated is local convention rather than principle, an act that is unremarkable in one setting can be a serious faux pas in another — which is why the phrase attaches so often to cross-cultural travel and to arriving in an unfamiliar room.
The phrase entered English in the 1670s directly from French: faux ("false") and pas ("step"), from Latin falsus and passus. The buried metaphor is a misstep in a dance whose choreography everyone else already knows. English kept the French spelling and pronunciation and the French plural — the plural is spelled identically to the singular and distinguished only in speech, where the final s is sounded.
Its nearest English neighbors divide the territory finely. A gaffe is usually a verbal slip made in public, and carries a whiff of the political. A solecism is a breach of correctness, originally of grammar, extended to manners. A blunder is any clumsy mistake, social or not. Faux pas holds the specific ground of etiquette.