It's mostly just a term that hardcore feminists use to portray masculinity as a bad thing.
In reality, humans can be toxic, regardless of gender. There are plenty of toxic women in this world as well; evil does not discriminate based on gender, race, religion, etc.
Referring to masculinity as "toxic" is in fact toxic in and of itself, because it creates a divisive "us vs them" mentality among men and women. We need to stop obsessing about our differences(gender, race, etc.) and start focusing on the fact that we are all HUMANS.
We don't need any -isms(including feminism) to tell us how to think and behave. All we need are love, respect, and good moral values(do unto others as you would have them do unto you, etc.)