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1.
Dependant on tone and context, knobshank can be used as a friendly or unfriendly term to insult someone who may have done something, or simply is, stupid.
Friendly: 'I can't believe how drunk you got, you knobshank.'

Unfriendly: 'That guy is a complete knobshank.'
by Bob October 31, 2004
 
2.
The length of a penis minus the bellend.
Colin Wilkinson of Plymouth is a knobshank.
by Jimmybay May 31, 2003