How would you define "toxic masculinity" ?

It's a mistaken view of what it means and what it takes to be a man.

For instance, a man who believes that he must always be strong and be a leader will often believe he cannot afford to show sadness, disappointment, vulnerability or weakness.
Since he's human and cannot not have these traits at least sometimes, it means that by not showing his feelings he is being dishonest, preventing his loved ones from knowing him as he truly is, and cutting himself off from love, and from being loved for his whole self. But this is the mildest form of toxic.
The usual version of toxic is a control freak who uses an arsenal of tactics to subdue, bully and control the people closest to him. He wreaks devastating harm and trauma on his dependents. Those who escape his influence are survivors.

Women can also be toxic - their weapons may be somewhat different, but still just as damaging.

a man shaming another man for not “acting like a man”

Men encouraging misogynistic behavior

Never heard of it. Toxic femininity on the other hand, well that happens once a month, in my experience.

Made up term

Circumcision starts it. Telling little boys it isn't okay to express their feelings perpetuates it.

Laughing at the weird kid in school who thinks hot dogs symbolize all that is evil in this world.. 😷

Being chauvinistic, sexist, degrading.

Simply being a male??????

It's one of those vague and poorly explained concepts that are thrown around by ideologues.