The penis of the ambiguous figure called no-face.
It all started in a skype call. mid-2009 when no-face flashed his penis in the conversation topic picture.
Everyone was enraged and surprised.
Thus, RP is associated with the ambiguous No-Face or someone else who has a picture of his penis luring a group into a false sense of security and then surprising them with No-Face's penis.
Exmaple 1: I just RP'd Kaitsu at class today.
Example 2: I got RP'd by Kevin today!