Green’s Dictionary of Slang

slither n.

counterfeit money.

1930		(con. 1910–20s) D. Mackenzie Hell’s Kitchen 120: Slither ... base coin.
1941		V. Davis Phenomena in Crime 252: Snide, slither, dough. Counterfeit coins. The moulds are called ‘ovens’.

In phrases

drop the slither (v.)

(UK und.) passing counterfeit money.

1935

Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 4: Dropping the slither (or slether): Uttering base coin.