Why was a threepenny piece called a Joey?

Answer 1

The Joey slang word seems reasonably certainly to have been named after the politician Joseph Hume (1777-1855), who advocated successfully that the fourpenny groat be reintroduced, which it was in 1835 or 1836, chiefly to foil London cab drivers (horse driven ones in those days) in their practice of pretending not to have change, with the intention of extorting a bigger tip, particularly when given two shillings for a two-mile fare, which at the time cost one shilling and eight-pence. The re-introduction of the groat thus enabled many customers to pay the exact fare, and so the cab drivers used the term Joey as a derisory reference for the fourpenny groats

Answer 2

The slang term "Joey" was applied to different coins at different times in different parts of England and for different reasons.

The Groat (or Fourpence) arguably became known as a "Joey" after a politician, Joseph Hume, successfully argued for the return of the Groat to foil the practice of cab drivers getting a larger tip due to not having the correct change.

When the Groat was later withdrawn from the currency, the term "Joey" transferred to the Threepence, for the same reason.