When a person is unwillingly forced to do something through sex.
"Do you want to buy me that necklace"

"Uuh no"

**Blacknailed**

"How about now?"

"O.K!"
by relic_052 September 6, 2009
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Using potentially embarrassing information to coerce someone into sleeping with you.
John: Did you hear about Sara?
Patrick: What happened?
John: I heard she spend all her graduation money on coke. I work for her dad. Should I say something?
Patrick: Only if you're trying to blacknail her.
by honest≠asshole January 16, 2012
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