An unexpected, very rapid punch to the buttocks - typically for retribution - most often achieved while walking up a staircase. "Ninja butt-punch" is quickly, quietly spoken by the attacker as he makes his move, with "BUTT" heavily accented as the fist makes contact.
"ninja BUTT-punch" -- "Ahh!! You faggot!!!"
by alftoid January 04, 2010