A man who has a masculine style, habits and a demeanor that is admired by other men. Someone who is not thought of as metro-sexual, perhaps.
And the broad consensus of who is one shifts. In the '40s and '50s, Hemingway was considered one. In the '60s, '70s and '80s it was Sean Connery, Warren Beatty or Paul Newman. Now it might be Harrison Ford, as an example.