when the cat's away, the mice will play; taking a break while the boss is out; doing what one wants when alone
Proverbial expression meaning to relax or do as one pleases when a feared or strict person (the 'demon') is absent. Often used humorously.
See also: 命の洗濯
The department head is away on a business trip, so today we can relax while the boss is out.
The kids were playing games while their parents were out, as if to say 'when the cat's away, the mice will play.'