Someone who's manly acts in a typically masculine way. If you live in a log cabin, split your own wood, and occasionally scare away bears, go ahead and call yourself manly.
Manly attributes are usually heroic ones like bravery and strength. Despite the fact that many women also have these characteristics, it's still common to describe them as manly. Manly activities might include fishing, hunting, and other rugged, outdoorsy sports. In the thirteenth century, this adjective simply meant "characteristic of a human."