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1.
A. - Immense pain you get when hit in the balls.

B. - A description for a really fit girl - i.e. she is giving you an erection, hence knobthrob.
Bloke 1: AARGH! Bloke 2: Ooh, that must be a real knobthrob!

Bloke 1: Damn, that chick is a knobthrob.
by Nuggenheim November 07, 2007