Any extreme of emotion; losing your cool in any way; being extremely silly or weird.
"HOW COULD THIS HAPPEN TO ME!!!"
Dude, stop lunchin, it's gonna be cool.
*uninhibited giggling nonstop*
Hahaha, you're totally lunching.
"Maybe we should try to write this report in pizza sauce!"
Yeah, no, dude, stop lunchin out and get to work.