Consider an old British scam involving something that glittered but was not gold.
Here's how it worked:
A con man would gild a brass ring to disguise it as gold, surreptitiously drop it, and then run to pick it up at the same time that an unsuspecting passerby noticed it on the ground.
The scammer would then propose that the found treasure should be split between them. The one who'd "found" the ring, convinced now of its value, would choose instead to keep the ring and pay the con artist some amount of money. That amount, of course, was a bargain for gold but a high price for brass.
That ring was called the fawney. Its fakeness probably gave us phony.