expression, godan verb (-su)
be exhausted; be worn out; be done in
Idiomatic expression literally meaning 'to stick out one's chin', used to describe being so tired that one's head droops forward.
徹夜で仕事をして、すっかりあごを出してしまった。
I worked all night and was completely exhausted.
山登りの後で、みんなあごを出していた。
After the mountain climb, everyone was worn out.