Green’s Dictionary of Slang

meathead n.

[SE meat + -head sfx (1), implying that solid flesh, rather than brains, occupies one’s skull]

1. [1920s+] (US) a stupid person; thus as a term of address.

2. [1940s+] a general term of abuse.

In derivatives

meatheaded (adj.)

[1940s+] stupid, foolish.