an acronym (pronounced as a single syllable) for "Gently Flicking the End of My Penis]". It can generally be exchanged in place of "joking" or "kidding" to impart a sense of incredulity.
Kevin: Dude, lemme buy you a beer?
Chris: Serious? You're not GFEMP, are you?
Kevin: Yeah, you got me...
short for gently flick the end of my penis meaning to tease someone with something you have no intention of giving them.
(First used in a popular online roleplaying game)
Is that for me are are you just gfemp?