A man whom by virtue of the authority and privilege vested in him by society feels entitled to preach or explain how the world works. Bonus, if he is speaking to women, whom he perceives to be naive and ignorant, about problems and concerns with which women have real life every day experience, and the man has NO fucking clue what he is talking about. This is usually done in a patronizing insensitive manner.
A mansplainer lectures on gynecology, menstruation or gender oppression to women.

Ex. I think women should...
The problem with women today is...
by Hemmatime & Bstillo January 11, 2013
Men who open there legs to see there little friend.
Amy was a mansplainer front of her friends , they were very disappointed.
by What is love ? January 10, 2020
Essentially when a man assumes a woman (sometimes a man) lacks knowledge in an area where she (or he) is known to be an expert
Me reading the other definitions of "mansplain": Wow, these all sound like they were written by men who have never experienced mansplaining themselves! Now, they're trying to explain what it is. CLASSIC mansplaining!!

2nd-year law student: I cannot believe Matt just mansplained me about the difference between prosecutors and defense attornies. This dude is referencing TV shows LMAO
by shhhhhhhlj January 27, 2018
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
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
When a man explains something to a woman in a patronizing tone as if the woman is too emotional and illogical to understand.
"She got all pissed at me for no reason!" cried Ira, "Accusing me of mansplaining."
Aziz shook his head. "Women! Right?"
by Babydoll75 March 7, 2019
When unsolicited advice or direction is given to a woman by a man in a condescending manner. The reasons typically being, the man does not believe the woman is capable of completing the task independently and must need guidance.
"You're not doing that how I would do it so I'm going to bother you with my directions and opinions even though I am in no way a superior and you did not ask for clarification. There is no way you are capable of doing this by yourself and must be ignorant so you have to rely on my constant guidance."

"uh, I think I got it.. thanks."
Mansplaining
by tybkylie December 14, 2018