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1.
Somebody who is deceptive, ignorant or otherwise unpleasantly cunning. Not a friendly, jovial term, a Sackhead is very much similar to a dickhead, in which said person is claimed to have a penis, or in this case a (Nut)'Sack' upon their head.
'Matt is a Sackhead'

'Only a Sackhead would do that'
by Mightybear April 11, 2006
 
2.
a person who is acting stupid, trying to be funny, or a wanna-be
Bob, was being such a sackhead, that everyone left the party
by ashanakwa December 29, 2009