Is that a fake gun, or is it the real McCoy? [=is it a real gun?]那是把假枪还是真枪？

These diamonds look like the real McCoy, but they could be fake.这些钻石看起来像是真的，但它们有可能是假的。

To help her play the role of a lawyer, the actress studied with the real McCoy. [=with an actual lawyer]这位女演员为演好律师角色，跟一名真正的律师一起学习。

Maybe he's the McCoy, and maybe he's not.他有可能是本人，也有可能是假冒的。