Green’s Dictionary of Slang

slow-witted, foolish.

1884		Atlanta Constitution Sept. n.p.: Her lunk-headed admirer has to turn squarely around, blocking up the side-walk [DA].
1890		(con. 1860s) W. Goss Recollections of a Private 228: This darned soft-headed, lunkheaded Joe here wanted to take me up and carry me like a baby!
1901		Warren Sheaf (Marshall Co., MN) 31 Oct. 4/2: Them lunkhead chaps will see [...] what gawky chaps will see.
1908		Frank Harris Bomb 240: Every mail I was more astounded by the lunkheaded bungling of the Chicago police.
1932		D.W. Lovelace King Kong 83: ‘That was a lunkheaded trick,’ Denham told him.
1949		Sat. Eve. Post 16 Apr. 143/2: Take it then, and be fast cleared out, for you’re a lunkheaded moon-calf [DA].
1978		E.S. Maggin Superman 212: This lunkheaded hero’s amazing lack of imagination.
1999		Guardian Rev. 27 Aug. 18: The incongruous juxtaposition of lunkheaded made guys and their knock-kneed shrinks.