Green’s Dictionary of Slang

acorn n.2

[supposed resemblance]

1. (US) the head.

[US]Ade Hand-made Fables 45: He got it into the Acorn that each Tract of Ground [...] was operated as a Gift Enterprise.

2. (US gay) the glans penis.

[US]B. Rodgers Queens’ Vernacular 104: glans penis; the acorn-like bulb forming the uppermost end of the penis [...] acorn.
[UK]J. McDonald Dict. of Obscenity etc.

SE in slang uses

In compounds

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

(US) of humans and animals, a runt, a weakling.

[US]in DARE.
[US]Weseen Dict. Amer. Sl.
[US]Randolph & Wilson Down in the Holler 222: acorn calf: n. A runt, a poor specimen, a weakling. Applied to human beings as well as to cattle.
acorn-cracker (n.) [such individuals allegedly eat acorns but note cracker n.3 (1)]

1. (US) an uncouth rural person.

[US]J.W. Carr ‘Words from Northwest Arkansas’ in DN III:i 68: acorn-cracker, n. Uncouth countryman.
[US]Berrey & Van den Bark Amer. Thes. Sl. §391.3: Rustic; Bumpkin, Acorn cracker.

2. attrib. use of sense 1.

film review at www.imdb.com [Internet] The acorn-cracker Humour on the one side balance [sic] with the hard life-deal of breezy Villagers on the second side.