Masculinity is whatever society wants to market it as in the current time as it is a concept and not an actual thing, as is "femininity" or "falling in love" or even "marriage". It doesn't mean anything besides whatever catchy rhetoric is attached to it.
It's really periodic/societal and is mostly in the hands of people who control social norms of that culture-sphere, media mostly since they make what shapes large-scale social norms
Just look at history. Men used to join the army and go to war to prove themselves and for some weird version of family honor, not because they couldn't do anything with their life and in need of quick cash (like most today). Today, masculinity can also be getting a 9-5 and coming home everyday to see your family. Only time and society changing made that happen. The former was not even more than 100-200 years ago, which is a short time if you want to discuss the complete social history of gender and therefore the world and all it's history. No man who salved away on a railroad, or who voluntarily went to war, or who hunted animals to feed his family would find any of us today to "masculine" in comparison to the men off their time.
So yeah, to me, masculinity is fluid (and mostly non-existent in reality), as is everything in life, because we make life to fit us, not the other way around. That's akin to throwing yourself in a bucket and who wants to do that.