Green’s Dictionary of Slang

1. (US) a mule or stubborn animal.

c.1910		in T. McCoy Tim McCoy Remembers 33: My other knot head broncs.
1920		DN V 82: Knot head. A bucking horse.

2. a fool.

1851		Times-Picayune (New Orleans, LA) 25 May 4/4: Mr Flatfoot — Duz yer know Old Knothead.
1865		Belfast Morn. News 19 May 8/1: ‘he wouldn’t carry me as far as that white rhinocerous carried you, old knot-head,’ retorted Ben.
1929		M. van Denbark ‘Nebraska Cow Talk’ AS V:1 57: If these prospective ‘he-men’ never attain skill in their riding and work, they are ‘knotheads.’.
c.1930		(ref. to 1898) N. Kimball Amer. Madam (1981) 274: A lot were knotheads, chowder-brains, who had to take their shoes off to add up anything over ten.
1936		R.F. Adams Cowboy Lingo 26: If he could not ‘ketch on’ to the work required of him, he was a ‘knothead’.
1941		‘More Tennessee Expressions’ in AS XVI:1 Feb. 447/2: knot head. Low intelligence. ‘Bob was a knot head when he went to school.’.
1944		P. Kendall Dict. Service Sl. n.p.: Knothead: an unintelligent Marine; knucklehead: a knuckle lower than a knothead.
1960		(con. WWII) R. Leckie Marines! 128: All right, you knotheads!
1967		(con. 1920s) J. Thompson South of Heaven (1994) 159: All right, maybe I was a knothead.
1971		T. Thackrey Thief 300: The milkman got out and started to tell me all the different kinds of a knothead I was.
1987		in G. Norris New Amer. Short Stories 274: That’s reality. Not some knothead saying you’re a homosexual.
1991		B. Hamper Rivethead (1992) 179: How many gold-diggin’ knotheads had spun the web of the ever-popular self-help book?
2003		K.V. Forrest Murder by Tradition 138: ‘No, knothead,’ Tora said, [...] ‘Those five fools on the Supreme Court.’.