When a man explains something to a woman, uninvited, about a topic she is either equally qualified or more qualified to talk on.

A male doctor explaining something to a female patient, for example, would not be mansplaining.

Examples: The guy who tried to tell Margaret Atwood about "The Handmaid's Tale", the guy who told Katie Mack, who has a PHD in Astrophysics, to "learn some science" because he thought climate change was a hoax, the man who tried to correct a woman about how to pronounce her own name correctly, an Englishman correcting a Welsh woman on her Welsh pronunciation.

See also Dunning-Kruger effect.
Look at this idiot trying to mansplain periods to women... he thinks women only have nine periods a year... wait 'til you hear his lecture on how to insert a tampon correctly!
by Jorgs August 2, 2019
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When someone "explains" something to another person about a topic they are already familiar with/understand. This is typically done in a condescending manner under the pretense that the speaker knows more about the topic than who they are speaking to, i.e. they are subordinate.
1) Explaining the rules of poker to a poker dealer
2)Repeating a point back to someone in your own way, even if they just said the same thing
3)Telling a person that you know more about something because of blank, even though they are a professional/have a degree in the field that you do not have

Sidenote: it is okay to know a lot about a topic and make this known in conversation, but assuming that you know more than someone else, not accepting if you are wrong, not accepting that they may simply be more knowledgeable, and/or doing so in a condescending manner means that you are mansplaining.

Note: this definition and examples were built together by both a man and a woman
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A way to attack faux sexism by using actual sexism
She said an El Camino was a truck. I told her I feel like it’s more a hybrid of a car and a truck.
She shot me for mansplaining and the woke majority called her queen
by CreativeFunnyName November 8, 2020
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A bullshit term used by supposed feminists (read: misandrists) to dismiss any logical and/or factual explanation by a man simply because they have issues with men.
Woman: "Men are more privileged than women in almost all areas of life."
Man: "That's not true. We're mostly alone and treated with contempt, nobody cares about our problems, and if we're useless for any reason, we're left to die."
Woman: "Please. Save me your mansplain of how you have it worse than us."
Man: ".... what?"
by Davis The Untamed July 19, 2020
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to delighting in condescending, inaccurate explanations delivered with rock solid confidence of rightness and that slimy certainty that of course he is right, because he is the man in this conversation
Even though he knew she had an advanced degree in neuroscience, he felt the need to mansplain "there are molecules in the brain called neurotransmitters"
by Handmaid February 5, 2010
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To explain in a patronizing manner, assuming total ignorance on the part of those listening. The mansplainer is often shocked and hurt when their mansplanation is not taken as absolute fact, criticized or even rejected altogether.

Named for a behavior commonly exhibited by male newbies on internet forums frequented primarily by women. Often leads to a flounce. Either sex can be guilty of mansplaining.
The members of the equestrian community were unimpressed when Bob posted an essay that mansplained a solution to a horsy problem they were all well-acquainted with.
by TheGiggler April 25, 2009
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An explanation provided by any man because women aren't capable of providing it.
After hours of listening to Mary babble, I mansplained our sales pitch in a few minutes and everyone loved it.

LaToya struggled with her dinner order, so I mansplained it to the waiter.

The traffic cop was about throw a beat down on Tanya but I mansplained that she was in labor.
by Al-BadDaddy November 18, 2017
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