Green’s Dictionary of Slang

jemmy n.1

also jimmy
[ety. unknown]

1. [early 17C] a fool.

2. [mid-18C] a wig.

3. [mid-18C–1910s] a sheep’s head [Bee (1823) suggests an actual butcher Jemmy Lincomb, who lived near Scotland Yard].

4. [mid–late 19C] a large human head.