Fly the coop

To ‘fly the coop’ is a metaphor for departing suddenly or escaping from confinement and is originally American from the early 20th century, although ‘coop’ was American slang for jail or prison from the late 18th century. It derives from the obvious allusion of poultry departing or escaping from the confinement of the coop or henhouse. See also Bird has flown.