A total butthead is a mix of the words "total" and "butthead".
Definition of total (as of dictionary.com): "(Adj) complete; absolute: the evening was a total failure" 
Definition of butthead (as of dictionary.com):"(N) Slang: Vulgar an extremely stupid or inept person." 
OR you could look at the slang definition (a of dictionay.com): "(N) a stupid or obnoxious person of either sex. (Also a term of address. Rude and derogatory.) : Don't be such a butthead!"
So calling someone a total butthead is like calling them an absolute stupid/obnoxious person.
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