An acronym for "I will find you and kill you," something you say to a motherfucking asshole when it pisses you off to the point when the only way to get them to knock it off is by a death threat
Motherfucking Asshole: You hate me, son! You ruined my life! I tried to have sex with you one time, and you called the goddamn police, fucker!!! God fucking hates you!
Decent Human Being Just Trying to Live: IWFYAKY
Motherfucking Asshole: (timidly) Nevermind.