The term "toxic masculinity" was coined in the 80's by Robert Bly and Shepherd Bliss, two New Age religious practitioners who used it as a marketing term to sell their books and retreats. These retreats involved wearing loincloths and dancing around in Native American headdresses. Bliss explicitly defined "toxic masculinity" as "a form of poisonous masculinity" and offered his blend of pop-psychology and New Age religious ritual as the antidote.
The term was later picked up by figures in gender studies during the 90's like Raewyn Connell and Michael Messner. This research was purely interpretive, and involved looking at statistics and making subjective, philosophical conclusions that weren't directly rooted in any semblance of scientific methodology.
Usage of the term became widespread and popular among internet and social media feminists around the time of the Me Too movement.
However anyone may feel about the validity of the term, it was never scientific in nature. It is a purely religious and philosophical concept. In recent discussions on this sub, lots of folks seem to have the mistaken impression that this term was based in science even where they understood it's New Age religious origin. At no point did the term somehow become scientific, which isn't even really possible in a case like this.