lights-out \ˈ ̷ ̷ˈ ̷ ̷\noun (plurallights-out) 1.: a command or signal (as a bell or bugle call) for putting out lights 2.: a prescribed bedtime for persons living under discipline (as in a boarding school or army camp) < the only place where he could hide to study after lights-out — H.H.Martin > < while away the hours before lights-out — English Digest >