Green’s Dictionary of Slang

jigger n.4

[? jigger n.1 or jigger n.5 (1) or SE jig, to move up and down]

1. the penis.

[UK]Farmer & Henley Sl. and Its Analogues.
H.N. Cary Sl. Venery 157: jigger—(2.) the penis.
[US]Berrey & Van den Bark Amer. Thes. Sl.
J.Meades Filthy Eng. 60: in my hand I’ll have my short arm, peter, porksword,..jigger, &c.

2. the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
H.N. Cary Sl. Venery 157: jigger—1. The vagina.
[US] in G. Legman Limerick (1953) 193: Her belly began to rise, / And out of her jigger / Jumped a cast-iron nigger.
[US] in Randolph & Legman Ozark Folksongs and Folklore (1992) I 133: Learned from a Negro in central Arkansas, about 1910. [...] ‘Went upstairs to get a little jigger, / She says wait till the hole gets bigger, um-hum’.