From Wiktionary, the free dictionary
From dip + -head.
diphead (plural dipheads)
- (rare, slang, derogatory, euphemistic) An idiot; a moron.
1992, D. A. Fowler, The Devil's End, →ISBN:“He's too young to learn tricks, you diphead.” “He ain't neither, donkey face."
2013 June 11, Jean Davies Okimoto, To Jaykae: Life Stinx, Tor Books, →ISBN:My name's Jason, you diphead.