When you're hanging out (usually with a bunch of people of the same sex) and you start acting weird because you are either tired or excited. For girls, an example could be laughing uncontrollably at something that isn't funny. For guys, an example would be rockin out to N'sync or Lady Gaga without caring about who sees you/being made fun of. In general, going crazy.
Dude, Joe's singing 'I kissed a girl' off-key at the top of his lungs.
--I know, he's nerping right now.