Refers to someone (usually a man) who, a lot like
the most interesting man in the world from
the XX commercials, purports to be able to have grandiose powers and do things that one else can. Can be both positive or negative depending on how the phrase is used. In the positive, it is a compliment to someone's ability; in the negative it is a
tongue in cheek statement, that's really meant to slight someone as not being capable of anything like he purports himself to be capable of.