“Flew the coop” comes from the incontrovertible fact that chickens are saved in hen coops, each for their very own protection and for the protection of the farmer’s assets.
Alternatively, from time to time there are a couple of brave cocks and hens in a pen, who try to break out – an incidence, that led to the introduction of the colloquialism “Flew the coop”.
It first began showing in print in the overdue 19th century, with a number of novels, studies and publications that includes the word by way of the 1890’s.