Green’s Dictionary of Slang

crumb n.2

[? the shape]

1. (US) the head.

[UK]‘One of the Fancy’ Tom Crib’s Memorial to Congress 16: But the Bear, up to slum, / Follow’d close on my gentleman, kneading his crum.

2. the penis.

[US] in E. Wilson Twenties (1975) 252: I guess we’ll have to call it the clap [...] I was just thinkin’ that must have been some puddle where you dropped your crumb!