Green’s Dictionary of Slang

pinhead n.

[SE pin + -head sfx]

1. in US drug uses.

(a) [late 19C–1930s] a small pill of opium, costing 25 cents.

(b) [1920s–40s] one who injects narcotics, esp. when using a rudimentary syringe based on a pin and medicine dropper.

(c) [1960s] an amphetamine user.

(d) [1960s+] a very thin marijuana cigarette.

2. [late 19C+] (orig. US) a stupid person.

3. [late 19C+] (orig. US) a person with a small head.