Green’s Dictionary of Slang

acorn n.2

[supposed resemblance]

1. [1920s] (US) the head.

2. [1970s+] (US gay) the glans penis.

SE in slang uses

In compounds

acorn calf (n.) [Western US belief that a cow that ate too many acorns produced weak offspring]

[1920s+] (US) of humans and animals, a runt, a weakling.

acorn-cracker (n.) [such individuals allegedly eat acorns but note cracker n.3 (1)]

1. [20C+] (US) an uncouth rural person.

2. attrib. use of sense 1.