A friend of mine was in line to watch the new Captain America movie back when it first came out, and some nerd behind her started explaining who Captain America was to her as if she didn't have a clue. She's read the comics since she was in middle school and probably knew more than this guy and even told him that, but he just rattled on.
Good example of classic mansplaining.
To be sure, we don't know why the guy started talking to her. Could be out of the assumption that his input was valuable to her. But it could also have been an excuse to talk to a woman. The key point is that, in some way, the male-female dynamic was why he launched into his condescending explanation.
It's actually understandable in some cases that a guy, even with only rudimentary knowledge of a subject, might want to display that knowledge to a woman. Though his knowledge lacks, he wants to put it all out there and give his best shot at displaying his worth. It is precisely because it is kind of understandable that he needs to be vigilant against it. Because even if he only did it to impress her, it doesn't make it any less annoying or any less silencing to the woman.
So mansplaining involves the male-female dynamic in some way being the initiative for the explanation, combined with the condescending/ignorant explanation itself.
Here we must decide if either factor alone should qualify. Factor 1 only: If Einstein stopped women on the street to explain relativity would he be mansplaining? Factor 2 only: if a woman assumed other women knew nothing of comics and launched into an explanation, would she be mansplaining?
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That brings us to phase 2 mansplaining. When a man explains topics such as pregnancy with ignorance, it may be reasonable to expand mansplaining to cover that even if the setting has no women.
In essence, we are considering the male-female dynamic
of the topic. If the male-female dynamic of a topic initiates an ignorant, condescending explanation, then even if a woman is not the recipient of the diatribe we could choose to call it mansplaining. That is, the dynamic of the topic lowered the bar for entry, where the explainer would not have given his explanation in another topic with the same level of knowledge about it.
Again we must decide if we want to relax any assumptions. Need we require that the male-female dynamic was instrumental in the decision to launch?
And that's where I think some people have problems with the term. On the one hand, it can make sense to relax that assumption: to a woman who hears an explanation, it can still come across as mansplaining whether or not the internal motivation had anything to do with gender dynamics. But if we relax that assumption, then mansplaining really does become identical to stupidity or wrongness when dealing with gender related topics. And that's problematic because mansplaining is a gendered term, so we would be associating a gender with stupidity and ignorance (again, if we relaxed the assumption).