According the New York Times, 'dickhead' is not a word

“Deadheading” is the cutting off of existing blooms which are past their peak in order to allow the rest of the plant to more effectively provide nutrients to other areas which have the potential for budding....it’s a common agricultural practice having nothing to do with the Grateful Dead.
Dickhead is obviously a word, but it’s a vulgar slang insult that the NY Times decides not to include in their wordbase.