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1.
A quick tempered, slow witted jock, often in the context of a bully. A Biff often compensates for his lack of knowledge with force or otherwise aggressive action.

The term is derived from the bully in Back to the Future I and II who harassed George McFly not only in has past, but also through the future.
Yo McFly, why don't you make like a tree and get outta here.
by Fred Rogers February 25, 2004
 
2.
The sexual maneuver in which a man uses the head of his erect penis to vigorously knock three times on his partner's forehead and then exclaim "Hello?! McFly?!", in reference to Back to the Future Part II.
"I Biff Tannen'd her so hard I put a dent in her skull and now there's blood when I urinate."
by ColonelSandersEngorgedCockHead December 06, 2009