Ready lights on

Nipples poking through a chick's shirt. Back in the day, B-52 bombers carrying nuclear weapons had a process whereby they would flip some switches to arm their bombs. Above each switch was a raised bulb that would light up when the bombs were ready to explode if dropped. A bombardier would say "Ready lights on" as a way to let everyone else know that he was ready to do some damage. The expression then came to be a way for aircraft personnel to call attention to nipples showing up the way ready lights would.