Green’s Dictionary of Slang

jemmy adj.²

rubbishy, nonsensical.

1890–1904

Farmer & Henley Sl. and Its Analogues IV 45/1: Jemmy (or Jimmy) [...] A term of contempt.

In phrases

all jemmy (also all jimmy)

all nonsense.

1890–1904

Farmer & Henley Sl. and Its Analogues IV 45/1: All jimmy = all rot.