From Wiktionary, the free dictionary
Literally, “when the cat isn't there, the mice dance”.
- IPA(key): /kɑ̃ l(ə) ʃa n‿ɛ pa la | le su.ʁi dɑ̃s/ ~ /kɑ̃ l(ə) ʃa n‿ɛ pɑ la | le su.ʁi dɑ̃s/
quand le chat n’est pas là, les souris dansent
- (idiomatic) when the cat's away the mice will play