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.
the state of mind where there is no longer a state of mind
usually happens late at night while your drinking energy drinks that contain tons of caffeine...the best nerp drink is Mountain Dew...you can tell your nerping when you laugh uncontrollably for no apparent reason...best nerp hour is usually 3am but you can nerp at anytime...usually you begin to nerp when you're really tired....you cant force yourself to nerp, it comes naturally...Silly in a manic sort of way, as in the frame of mind induced by too little sleep and too much caffeine