1. The prisoner waited until the guards were fast asleep. He unlocked his cell door with a hairpin and flew the coop.
2. The man felt trapped in a dead-end job. All he wanted to do was fly the coop.

The expression is usually used in connection with being in jail (opt.1) but can also be used figuratively (opt.2). It is a reference to captured birds escaping when their cage (coop) is opened.