Green’s Dictionary of Slang

lob n.3

also lobb
[SE lob, something pendulous]

the penis, esp. when half- or fully erect.

[UK]Farmer Vocabula Amatoria (1966) 46: Boyau ridé = a worn out penis; ‘a lob’.
[US]E. Field ‘A French Crisis’ in Facetiae Americana 20: Let other kingoes, lobs and yards, in adoration drop.
[US]H. Selby Jr Last Exit to Brooklyn 47: Harry [...] tolder he got a nice fat lob forer ta suck.
[US]B. Rodgers Queens’ Vernacular.

In phrases

on the lob (adj.)

of the penis, semi-erect.

[UK]‘Farmer’s Daughter’ in Cabinet of Venus 80: His staff was [...] on the lob, in a sort of ‘half cock’.