It is a blended word from man and explaining, and is a pejorative term.
Mansplaining — explaining without regard to the fact that the explainee knows more than the explainer, often done by a man to a woman and with the assumption that the woman does not already know what he is telling her.
It is a concept created by Rebecca Solnit in the early 2000s in an essay entitled Men Explain Things To Me. Solnit didn’t use the word mansplain but she explained it, describing the time a man explained a book to her without acknowledging that she herself wrote it.
A month later the word appeared in a comment on the social network LiveJournal. It became popular among feminist bloggers before entering mainstream commentary. The word was included in 2010 by the New York Times as one of its words of the year.
Women experience mansplaining in a wide variety of fields, the workplace, personal life, social media, etc. The concept describes a phenomenon that involves an overconfident man who explains something, usually to a woman, in a way that feels patronizing and condescending. The most heard sentence is “let me explain to you…”.
Even if the word is a neologism, the phenomenon is quite old and it is deeply rooted in our societies, to the point that some people don’t even recognize it when it happens. An example of this is a man lecturing a woman about menstrual cramps.
Sometimes mansplaining appears in more indirect forms, like a constant interruption, or talking over. It is considered a subtle manifestation of sexist conduct.
See more resources
Bridges, J. (2017). Gendering metapragmatics in online discourse: ‘Mansplaining man gonna mansplain…’. Discourse, Context & Media, 20, 94–102. https://doi.org/10.1016/j.dcm.2017.09.010.CrossRefGoogle Scholar
https://www.dictionary.com/browse/mansplain
https://www.merriam-webster.com/words-at-play/mansplaining-definition-history
https://www.theatlantic.com/sexes/archive/2012/11/a-cultural-history-of-mansplaining/264380/