You almost never meet McCoy by itself in this meaning. It lives inside one set phrase: the real McCoy. That means the true thing, not a copy, not a cheaper look-alike, and not a story someone dressed up. People say it about things they care about being genuine — a watch, a bag, a guitar, a signed shirt, even a recipe. If a stall sells a “famous” bag for a tiny price, that bag is not the real McCoy. The phrase is a little old-fashioned and a little playful. Younger speakers often just say the real thing or genuine, but you still hear McCoy in films, books, and from older speakers. It is informal, not a shop label. Though actually, nobody agrees where the phrase came from. Some people tell a story about a boxer, some about whisky, some about an inventor. Those stories are tidy; the history is messy. What has stayed is the feeling: this one is the true one, and the others are pretending.