Green’s Dictionary of Slang

cob n.4

[orig. dial. cob, a cock or male swan, cob, to make hay into cocks, or various uses that mean the head or top]

(US) the penis.

[US] in P.R. Runkel Law Unto Themselves 133: I’d never so much as stuck my cob in a girl’s shuckings.
[US](con. 1920–57) Randolph & Legman Ozark Folksongs and Folklore II 787: Other common names for the male organ are cob, [etc.].

In derivatives

cobbing (n.)

sexual intercourse [dial. cobbing, a thrashing].

[UK]‘Cobbing a Stiff-Un’ in Flash Minstrel! in Spedding & Watt (eds) Bawdy Songbooks (2011) I 111: Come, feel how very stiff it is, / You may ease it by rubbing, / But if yon’ll [sic] consent, I think, / There’s nothing like a cobbing.