v./n. inf(!) to unexpectedly penetrate someone's anus; often, of a heterosexual couple engaged in doggy style intercourse, when the male withdraws from the vagina, and hilariously re-inserts into the anus without prior warning.
NOT to be confused with the equally amusing Cadbury Surprise.
Betty: ... but then, as I... as I was about to... come, for some reason he pulled out... and then... *breaks down sobbing* he.. he put it... oh god! *sobs uncontrolably*
Psychiatrist 1: He gave you a BACKDOOR SURPRISE!? What a guy!!!
*hi-fives Psychiatrist 2*
Psychiatrist 2: Man, that is awesome!