Green’s Dictionary of Slang

asshead n.

also arsehead, asshat
[ass n. (1) + -head sfx (1)]

(US) a fool, an idiot; thus ass-headiness, foolishness.

[UK]Becon Early Writings n.p.: [A fool is called] asshead [F&H].
[UK]Bale Apology 61: O absolute ass-heads [...] and wytlesse ydyote [F&H].
[UK]Hay any Work 36: As verye an Assehead as John Catercap [F&H].
[UK]Shakespeare Twelfth Night V i: An ass-head and a coxcomb and a knave.
[UK]J. Minsheu Ductor in Linguas n.p.: Asshead, or blockhead. [Ibid.] Asse-headdinesse or blockishnesse.
[UK]Rowley Shoo-maker, a Gentleman Act IV: I smell an Asse-head of your own.
[UK]Morn. Chron. 9 Mar. 2/2: The one should call the other ‘an asshead’.
[UK]London Standard 24 Mar. 3/2: Here is the Asshead gaping.
[US]Portage Sentinel (Ravenna, OH) 7 Jan. 1/2: Col. Gristle didn’t relish it much to hear his sider called a asshead.
[UK]E.V. Kenealy Goethe: a New Pantomime in Poetical Works 2 (1878) 336: Ass-head, Blackmoor, Cuckoo, Dotard, / Splay foot, Yelper, wry-necked Wretch.
[WI]L. Bennett ‘John Cunnoo’ Jam. Dialect Poems 22: Go dung King Street an meck we meet / Nuff ass-head an John cunnoo.
[US]A. Lurie Nowhere City 26: ‘You know what I want, ass-head,’ Glory said.
[US]N. Mailer Why Are We in Vietnam? (1970) 143: So, ass-head America, contemplate your butt.
[Aus]P. Temple Black Tide (2012) [ebook] Same problem them if there’s two of you or fifty-two testosterone-crazed arseheads with guns.
[Aus]P. Temple Dead Point (2008) [ebook] Like you get to the big-time by being such an arsehead that the delivery boys can take the Feds to you.
posting at 3 May [header] And Hate Guy is a complete asshead.
[US]Eble Campus Sl. Nov. 1: asshat – someone stupid, mean, or annoying.
[US]D. Winslow The Force [ebook] Asshats who think that the answer [to crime] is to arm every one.

In derivatives

asshattery (n.) (also arsehattery)

stupidity. 3 Mar. [Internet] What I cannot defend, however, is asshattery in the name of grammar.
S. Saracino Life after Death [ebook] I really don’t have time for this, so how about you knock off the asshattery now and get up and cooperate like a big boy?
Twitter 3 Apr. [Internet] Which histrionic, ahistorical, arsehattery might well sum up the international laughing stock the Tories are turning us into.