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1.
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!
by Terry Deary February 24, 2006
 
2.
the act of reaching ones hand around anothers bottom and sneaking an object in said butt hole/crack
last night i totally hit my gf with a back door surprise, she loved it
by butt grabber11 March 03, 2009