Green’s Dictionary of Slang

jemson n.

also jemmison
[? SE jimsonweed, i.e. the penis also grows]

(US, Ozarks) the penis.

[US] in V. Randolph Pissing in the Snow (1988) 91: Then he outs with his old jemson, and it sure was a dandy.
[US]Randolph & Wilson Down in the Holler 100: I have heard a term for penis, which sounds like jemson or jemmison.