PJ down

'To PJ down' means to get changed out of one's pyjamas or just to get naked in general

"Hey, I'm just going to go and PJ down..." = I'm just going to go and get changed out of my PJs

"Hey, I just PJ'd down" = I just got changed out of my PJs

"Hey, you wanna PJ down?" ;) = "Do you want to get naked?" ;)