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1.
A celebration by subordinates on the occasion of their supervisor's absence. Usually applied to office workers, but relevant to any mischievous celebration orchestrated by peers answerable to a common individual.

Borrowing from the phrase 'when the cat's away, the mice will play', a mouse party might be celebrated by a team of employees taking a two-and-a-half-hour lunch together, or sneaking out of the office an hour early to drink at a bar.
Frank's going to be "working from home" tomorrow -- who's planning the mouse party?
by Luxcrete August 15, 2011