Yesterday, I ran across this bit of legalese for the first time, so I thought I'd note it -- it's Law French for "next friend," which is to say (to quote Black's), someone "who appears in a lawsuit to act for the benefit of an incompetent or minor plaintiff ...." The phrase, I'm pleased to say, is used only about 0.5% times as often as "next friend," but that still includes 20 cases (found via a Westlaw ALLCASES search) since 2005. So don't use it, but know it.
By the way, the phrase isn't French-by-way-of-Louisiana, but rather apparently French-by-way-of-1066 (or in any event by way of French influence on early English law). Of the 20 recent references, 3 were from Maryland, 6 from Connecticut, and 7 from Hawaii. It's also occasionally spelled "Prochein Amy" or "Prochain Amy."
Special bonus opaque legalism, which is noted in the question posed below: Some courts in New England states use the phrase "ppa" in case captions; this means "per prochein ami," or "by next friend." It's commonplace in Connecticut, not uncommon in Massachusetts, and seen occasionally in neighboring states.