Green’s Dictionary of Slang

slither n.

[it slithers through one’s fingers]

counterfeit money.

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