What is a smasher?

Solution 1:

A smasher is a person who puts counterfeit coins into circulation. Here's the relevant definition from A Dictionary of Archaisms and Provincialisms (1855):

Smasher
a passer of counterfeit coin.

(This definition was in earlier sources too; the Oxford English Dictionary cites it as being in the 1795 edition of A new dictionary of all the cant and flash languages, both ancient and modern.)

Solution 2:

From Green’s Dictionary of Slang

Smasher (UK Und.):

(also bit(t) smasher) one who makes or passes counterfeit money.

1796 [UK] Proceedings Old Bailey 6 Apr. 443/2: Q. Were you not taken up on suspicion of being a smasher? – A. I do not know the meaning of the word. Q. Upon your oath, do not you go about with a bag, and cry ‘any bad shillings’? – A. No. [...] Q. You would be surprized, if I were to tell you a smasher means a putter off of bad money?

The term derives from the noun smasher meaning

(UK Und.) counterfeit money [? it smashes the hopes of those who use it].1790.