Comes from old times, when the butt flap was still popular.
Note! There must be no consent for this to work properly.
Jack: Yeah, I pajama'd her and she didn't even make a noise!
Brit: I'm wearing pyjamas to bed (means he's going to bed in clothes specifically for sleeping. Probably with Chuck Norris or Superman on the front. Or Scooby Doo. I used to love Scooby Doo back in the day. Its still pretty good but you need to find a decent excuse to watch it - like having a little brother or something.)