Often defined as aggressive, strong, and unfeeling or stoic. Being masculine means in modern times, at least, no shows of emotion, no flamboyance, no hugging or even looking at other men, must be interested in sports and physical/violent activity.
What masculine is is simply what a man does. Assuming responsibility for your actions; being sympathetic, sensitive, and caring; and gentleness are perfectly fine for men. A true man is not a hard-nosed, violent, overgrown child; he's a mature, kind individual.
Until the 1800s, men's dress was about as flamboyant as women's dress, and men were allowed to show emotion and tears. But their clothing styles sobered and they were taught to be strong, stoic. Affection was still allowed... but as the world became aware of gays and other sexual minorities, male/male affection was scorned.
Positively characterized as qualities that are; strong, community building, utilitarian, practical. Negatively characterized as too aggresive, violent, uncaring.
I'd like to see the word masculine be used in good context for once