Every summer my extended family gets together for a week at the beach. You would think that with most of them being academics, we could pick pretty much any week in the summer and they could make it. This turns out not to be the case. In a break from tradition, both my nieces (both the one with the Ph.D. in chemistry and the one with most of a Ph.D. in musicology) managed to make it for the full week.
This was most fortunate, as it gave them to opportunity to benefit from the wisdom of their aged uncle. So when the topic of mansplaining came up, I graciously offered to explain it to them. They, being well-bred young ladies, cast their comely visages toward me, with their innocent eyes wide and with expressions of delighted gratitude. “Oh dearest uncle!” they cried, “Do, please, bestow upon us the benefit of your experience!” Such treasures must be shared, so now I will explain mansplaining to you, too.
Mansplaining is usually understood to be some guy explaining something to some woman, regardless of their respective qualifications in the subject. This is true so far as it goes, but it misses the essential point. This isn’t just something that guys do to women. We do it to each other, too. Not every guy. Some few poor benighted souls lack the mansplaining gene. But 73.6% of guys have it.  This is amply enough to make it a thing.
Why do we do it? For the pleasure it gives: for the frisson. The mansplaining need not be extended. Just having some visitor ask where is the men’s room will do it. Then there was the time that a guy wandered into the office because he knew he was looking for a law office on the floor, but couldn’t remember which one. I asked a few questions about what sort of legal matter he was involved in, and figured out from that which of the three law offices on the floor he was aiming for, and walked him to it. This was mansplaining nirvana: doubly so, since I got to mansplain to him, and then mansplain to the receptionist at the right office. That made my whole week!
But might not who or more guys try to mansplain at each other simultaneously? Of course! It happens all the time. When this occurs, it becomes a competitive activity. This explains every sports and/or politics argument ever made, and indeed most of the internet. This also explains the notorious reluctance of guys to ask directions. That is pretty much entering into the competition by lying on your back and exposing your throat.
Competitive mansplaining need not be a bad thing. Quite the contrary: correcting another’s factual error is a particularly satisfying form of mansplaining. This gives an incentive to get your facts right. This often leads to the memorization of a bunch of useless trivia. No competitive mansplainer wants to be caught getting wrong the first appearance of Spider-Man  or how many home runs Babe Ruth hit . Indeed, competitive mansplaining leads to systemic progress in human knowledge. A minor example is modern advanced sports statistics. Nowadays they are used to gain a competitive advantage on the field, but they originated from guys who wanted better weapons for arguing whether Cobb or Mays was the greater player . And really, what is the scientific method but a way to get the upper hand at mansplaining? Isaac Newton was the greatest mansplainer since Moses.
The problem arises when mansplaining is non-competitive. Some mansplainers surround themselves with non-mansplainers, thus allowing their mansplaining to devolve into bullshit . Some do this through personal charisma. I imagine that Christopher Hitchens was one of these. Whenever I read him, I pictured him at a cocktail party with a drink and a cigarette, expounding bullshit to a circle of rapt admirers. More common is to use a power differential to achieve unencumbered mansplaining. Donald Trump has the financial resources to ensure that he is surrounded by persons either lacking the mansplaining gene or willing to suppress the urge for financial benefit.
This brings us back to man-on-woman mansplaining. The traditional power differential across the gender divide made women an easy target for bullshit mansplaining. We are only talking about it now that the womenfolk have gotten uppity.
My nieces are good girls. “Thank you, most learned uncle!” they exclaimed. “It all makes sense, now that you have explained it to us.”
At least that is how I remember it.
 You can look it up.
 Amazing Fantasy No. 15.
 714. There is an argument for 715, but it is deeply misguided, deserving more of pity than scorn.
 It is close, but the correct answer is Mays. He also had the better nickname.
 In the technical, Frankfurtian sense of the word.