Cocqcigrues

Robert Hendrickson, QPB Encyclopedia of Word and Phrase Origins, 2nd edition (New York: Facts on File, 2004), s.vv. when the Cocqcigrues come (p. 771):

In French legend the Cocqcigrues (kok-se-groo) are fantastic creatures unlike animals anybody has ever seen. Thus when the French say A la venece des Cocqcigrues ("at the coming of the Cocqcigrues") they mean "never." The same meaning is conveyed in English with When the Cocqcigrues come. The word is sometimes spelled Cocquecigrue.

The French never say A la venece des Cocqcigrues; they say A la venue des Cocqcigrues. There is no such word as venece in French.

Apparently Rabelais invented the expression (Gargantua and Pantagruel, chapter XLIX: à la venue des cocquecigrues).