Internet thug "bootcamp" for assholes. Those who have the biggest e-peen are better than those who don't. People compare e-peen sizes via some odd sort of "flexing" that occurs in which people try to show the other how much better they are by using all capital letters.
Mary Poppins: I CAN FLY MOTHER-FUCKER.
Joe: DAMN YOUR E-PEEN IS HUGE.
In this case, Mary Poppins is showing how he is a master at e-peen flexing.