can someone explain to me this term "mental masturbation"

Masturbation is like sex with oneself, meaning there is no point to it other than self pleasure. It provides nothing to the world: no result from the action of procreation, a waste of creative potential. Mental masturbation, therefore, is thinking for the sake of your own stimulation. You do not use your thoughts to influence others or bring new ideas into the world, but, rather, you impress yourself by your own analysis and wit.

People use "mental masturbation" to mean different things. That's why it's important that you share the context, so we can reason what the autor actually meant. The most common meaning is "thinking and reasoning for the sake of having fun and not to be useful". When I read it or hear it it's being used negatively. It's used to say "that activity is useless". For example, you could argue that if you met a friend, and started talking, thinking and reasoning about the "trolley problem", and you both already know the problem, and you don't intend to study it nor prepare a any kind of study or research on it, (you are just having fun, having a laugh) that's mental masturbation. Just for example, someone hearing you could insult you by saying "why don't you both go get a job or something? you all really like engaging in mental masturbation".

I've been watching some productivity videos lately, and I think "mental masturbation" in the context of productivity is that you're doing things that *feel* productive but aren't as productive as you thought. An example would be wasting hours organizing your email or making a detailed planner. They feel productive, but the time you spend doing them is better if you have done other things. So instead of taking an hour to organize your email, make it 5-10 mins only, then move on and do something else. Some of the answers here are actually pretty good. I don't know what's OP looking for.

Thinking "one day I'll have succeeded" or thinking through all the possibilities gives you some sort of dopamine.  The concept of mental masturbation is that the only thing you're doing by thinking and not acting is getting that small dopamine hit, similar to the difference between masturbation and sexual intercourse. Sure, it feels good but it's not the same thing. 

Since you don’t like the answers so far, I’ll try..? Mental masturbation is what self-righteous weirdos do. They sit there impressed with their own analysis and thinking skills, satisfied with their “superior intellect.” It’s something Jordan Peterson (as mentioned in another comment) does a lot of. Anyone who considers themself an “intellectual” will likely partake in mental masturbation.

Mental masturbation is a derogatory term to describe deep and intensive thought without no practical utility.

That definition below is quite good. Masturbation is considered to be something that gives *you* pleasure, but doesn't achieve anything. So mental masturbation is thinking about things, without it being helpful. It's usually used as an insult, about what other people are doing - if they are "wasting time" on unproductive thoughts. For example: If you could travel in back in time, who would you want to meet? Maybe Newton, or Jesus, or perhaps you want to kill Hitler... we could discuss it for hours. But others might call that mental masturbation, because it's never actually going to happen; it's not possible - so, *some* people think that talking about it is a waste of time. Or, let's say I have to do an English TOEFL exam next week. I decide to carefully plan out my timetable, to make the best use of my seven days. I work out a system of colour-coding, and what I will do in every lesson slot, and in every break. I plan to do one hour on vocab each day... except all of this planning has taken me a day, so now I've only got six days, instead of seven. So I rearrange my plans - I fit all of the same things into six days. But doing all of that takes a day, so now I only have five... and so on. Mental masturbation. Not productive. https://www.youtube.com/watch?v=v-hg8nqN5K4

It simply means unproductive or pointless thinking that leads nowhere.

I’ll also add (for learning purposes) that, in your original post, you said that you came “through” this term. The correct preposition is “across.” People come across things, meaning to find things by chance.

I would interpret it as someone showing off how clever they are without actually contributing anything useful.

"Mental masturbation" is a slang term used to describe thinking or talking about ideas in a way that's intellectually stimulating but ultimately unproductive or self-indulgent. It often refers to: Overanalyzing things without taking action Engaging in abstract debates or philosophical discussions that have no real-world application Intellectual posturing—sounding smart for the sake of ego, not insight In short, it's when thinking becomes more about personal gratification than solving problems or making progress.

This thread has hit its happy ending, so I’m going to go full Locktober on this and finish it off.

Can you give the sentence for context? My guess would be that they are implying that the person is being self-indulgent

It essentially is thinking stuff like “Wow, I am so amazing!” Or “I am so fucking cool!” to the extreme, like when self-positivity goes out of control or takes on a sexual tone, I guess.