verb/ʁi.me kɔm al.baʁd e mi.ze.ʁi.kɔʁd/
to make no sense, to have no rhyme or reason
Synonyms: ne rimer à rien, n'avoir ni queue ni tête
Literally, “to rhyme like halberd and mercy”. Said to go back to an 18th-century epitaph where this rhyme, which is highly impure at best, was used. The effect is increased by the fact that the two words also have rather opposed meanings.