When the cat's away
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When/While the cat's away (the mice will play).
something that you say which means when the person in authority is absent, people will not do what they should do Do you think it's wise to leave the children alone for so long? You know, while the cat's away...See let the cat out of the bag, look like something the cat brought in, Look what the cat's dragged in!, play cat and mouse
See also: away