Whether you're talking about your attitude, a gigantic diamond, or that obnoxious guy's claims that he knows everything about quantum physics, you can use phony to say that something’s not real.
Use the adjective phony to describe anything that’s fake. If you’re making phony money in the basement, you’ll be in trouble if you get caught using it. Phony has a noun version with a similar meaning. If you say someone or something is a phony, then you’re saying it isn’t what it pretends to be. Someone who’s a phony isn’t sincere, like your classmate who pretended to be your best friend just to try to steal your boyfriend.