If you enter an amateur baking contest pretending to be self-taught when you're actually a French-trained pastry chef, you are a ringer. A ringer is a contestant who lies about his experience.
The idea of a ringer first developed in horse racing, where fast horses were sometime substituted, secretly, for slower ones — this was originally known as a "ring-in." It's thought to have come from the British term of the same name, which means "substitute or exchange." Ringers are occasionally used in many different sports and competitions, in an attempt to gain a sneaky kind of advantage.