Pas devant les domestiques is a
French reprimand that translates to "Not in front of the servants" in
English. One imagines it being uttered by a coquettish
deb brushing off the advances of her bewigged
paramour with a blush toward the
butler. A similar turn of phrase is the
pas devant les enfants, or "not in front of the children." This latter has the added charm of rhyme and is a little less
snooty.
Though I could find no corrobrating evidence, when I learned the term I was told it came from England of long ago when the "elite" could speak French but certainly not the servants, who were notoriously given to gossip. So if there was any behavior you didn't want propagating through the ServantNet (and thereby to other members of Polite Society vulgar enough to jack in), you needed to curtail the offending behavior until it could be conducted in true privacy. To avoid calling attention to the behavior via the reprimand, the speaker issued it in French.
In modern usage the domestiques in question can be any person or group perceived as lower in status than the speaker and listener. Certainly they shouldn't know French. One would hope it would be used tongue-in-cheek as a critique of the very class-conscious mindset which gave rise to it. However, I have personally overheard the term used by a catty and soulless drama queen at a cocktail party as an insiders-insult about a young, and (ok, I admit it) dumb twink that had stumbled into the wrong circle of conversation. So I suppose it is still being used to reinforce self-perceived exclusivity.