An acronym meaning : Please Tell Me If You're Coming Or Not
Typically used by people to replace the French term RSVP (Respondez S'il Vous Plait) which only means please respond.
Chuck: *answers phone* Hello?
Tom: Yo, I'm calling to tell you I will absolutely never go to your birthday party!
Chuck: Rude much, you only needed to call me if you were gonna come.
Tom: Naw, you said RSVP, if that were the case you shoulda said PTMIYCON
Chuck: Tom. . . STFU you literal bastard. *Hangs phone up*