1) This is the act of having sex with a girl (skank) and your junk slips out and slids in the ass. The only thing is you don't realize it and she has to tell you.

2) baning a chick and having you cock go into her ass, but her ass is so gapping that you don't even know you slipped up.
Karl: "I was going to town on her and all of a sudden the Saginaw Slip up!" "I stopped there."

Jimmy: "HUH, Saginaw what up?"

Karl: "Dude, she had to tell me my dick was in the wrong hole."

Jimmy: "Oh, the Saginaw Slip Up, I've done that to your mom."
by cheese4green September 22, 2011
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