An idiom meaning taking advantage of the absence of an authority figure (a boss, parent, etc.) to relax or do as one pleases; similar to 'when the cat's away, the mice will play.'
先生が出張でいない間、クラスはまるで鬼のいぬまに洗濯のように自由だった。
While the teacher was away on a business trip, the class was completely free, like 'when the cat's away, the mice will play.'
両親が旅行に行ったので、子供たちは家で鬼のいぬまに洗濯をしていた。
The parents went on a trip, so the kids were doing as they pleased at home.
店長が休みの日は従業員がリラックスして、まるで鬼のいぬまに洗濯だ。
On the manager's day off, the staff relax as if the boss weren't there.