"I just slaughtered 28 people in that game of Battlefield 2 and never died. Man my e-p33n feels huge."
peen - short for penis
A person's ego, pertaining exclusively to their online personality.
e.g. a big e-peen = a big online ego.
Unlike rl-peens (i.e. a real life penis), e-peens are 'flexed', brandished, whipped around, (or otherwise used) with unfailing regularity.
Another difference between e-peens and rl-peens are that e-peens can shrink and grow exorbitantly, i.e. more than a regular penis would with an erection.
Behavior quite commonly found on online discussion boards or similar, where some looser tries to establish his superiority - and thus the merit of his or her opinion - by stating how skilled and awesome he or she is, rather than forming a sound argument.
Generally also used to condescendingly refer to someones ego.
Rival: Don't pay any attention to the looser with the huge epeen!
casual_gamer: There's always some asshole e-peening on this forum <ignore button>.
Or just EPEEN. A joke made by Blizzard on April Fools 2010. Later made into an actual add-on you can use in-game. It measures your gear and displays your EPEEN bar above your characters head as well as measuring the surrounding character's EPEEN. The higher your EPEEN score is; the bigger the EPEEN bar is.