Douglas Harper's Etymology Dictionary
pis aller
"last resource, what one would do at the worst," 1670s, French, literally "to go worse," from pis "worse," from Latin peius, neuter of peior "worse" (see pejorative) + aller "to go."
Wiktionary
pis aller
n. A last resort.
WordNet
pis aller
n. French for `worst going' [syn: last resort]