Thieves and swindlers are common sources for many novel words as they engineer language to keep their secretive deeds under the radar. One such word is phoney, meaning ‘fake, not genuine’.
There are some false etymologies out there, including the theory that it has evolved from funny or is related to telephone in some way. However, the most plausible origin by far comes from American slang in the late 19th century.
Phoney probably started out as fawny which in turn came from fáine, an Irish word meaning ‘ring’. A fawny ring was used in a common fraud, which was known as a fawny rig or going on the fawny, in which conmen would sell gilt rings pretending that they were real gold. In the Classical Dictionary of the Vulgar Tonngue in 1796, Grose wrote:
FAWNEY RIG. A common fraud, thus practised: A fellow drops a brass ring, double gilt, which he picks up before the party meant to be cheated, and to whom he disposes of it for less than it is supposed, and ten times more than its real value.
With time, the word developed from meaning specifically a fake gold ring to denote anything that is not genuine.