My current working concept of this term is as follows.

Nice guy is essentially a man who has taken on board the heavy feminist gender roles imposed on men. ie stuff like men should do this, men should do that, men should treat women like this, etc. Specifically this would include the various double standards feminists demand of men, being subservient towards women, letting them get away with anything, endorsing any feminist claims about how nasty men are. More or less traditional conservative (Victorian) attitudes minus any self respect.

This is a hard line gender role feminists impose on men (while saying they oppose gender roles of course). Nice Guys are therefore in a sense feminists or feminist allies. They get told that if they fulfill their gender role they will get the usual rewards (after all that's what is true of any gender role) which used to include respect (not any more) but at least still include access to women.

Men: you must treat women with "respect". That means take on feminist imposed gender roles for men.

So a lot of guys do this and they don't get anywhere with women romantically. Now I'm not going to get into how much the gender role causes them to fail romantically, or whether the causality is different like men who do badly with women are more likely to accept or be subjected to feminist gender role enforcement, or whether both are caused by some other aspect. I don't even have to ask at this point if "Nice Guys" are indeed going to finish last with women romantically. But a lot do, so naturally they ask, what am I doing wrong? I did what I was supposed to do (gender role = feminist doormat), but I see girls going for guys who are failing to treat them with "respect". So what's up with that? Is what I have been told wrong? At this point they go back the the female feminists and grumble.

Well as you can imagine being feminists themselves their fellow feminists treat their concerns with the utmost respect. LOL. Not a chance. No, they double down. The "Nice Guy" gets told they are a huge shitlord for questioning feminism. That's disrespecting women! Somehow... ... make up some shit to justify that. OH here we go, let's say that anyone with this complaint clearly feels entitled to women's bodies.

This is pure bullshit justification for attacking their own allies. It's a ridiculous explanation even as feminist explanations go. These poor guys are the exact opposite of entitled when it comes to relations. They have no confidence and are constantly worried they are offending any woman they might have a thing for, with their shitlord "male gaze' etc etc. They're the exact opposite of entitled. But feminist justifications don't have to make any sense so it's stuck.

I'll quote some dude from MensLib where they recently tried to discuss this, ha.

As a former "nice guy" I'll throw in my two cents.

I keep hearing how people keep stating that "nice guys" feel that they are entitled to get women. This was not my line of thinking about this. I was a shy person and my friends and family would always say something along the line of "You're such a nice guy. You're going to make some girl happy one day." This led with the belief that I was not suppose to talk about my feelings or relationships with any led me to believe that being nice was the main, if not only factor, for getting a girl. Now, when this wouldn't happen, I would talk to a few people about it and their response was usually along the lines of "Just be a nice guy and they(girls) will like you."

Now image that this keeps getting repeated and repeated and you keep following this advice, but NOTHING EVER HAPPENS. You begin to get angry. You're doing what people are telling you to do, but you're not getting the results they said would happen. And this keeps happening over and over again. When ever I would complain that "Why does she going after that asshole? I'm so much nicer than him" it wasn't me thinking I should get her. It was me saying, "I've been told that being nice is how you get girls, but I'm not seeing this. Why is this? Why is what people have told me to do is not working and the opposite is working for this other guy."

So the root of the concept is feminist gender roles imposed upon men, and the consequent feminist vicious attack on those men that believe them (who ironically are mostly feminist allies), but then come to see it's bullshit.

A guy asking feminists for dating advice is like a black person asking the KKK how to register to vote.

The second effect behind the Nice Guy theme is just that feminists hate all men and don't like the idea of there being any such thing as a "Nice Guy" compare with an expression like "Good Indian" as in "The only Good Indian is a dead one". Clearly feminists hate the idea of a man being good so the best thing to do is to claim that even the best men out there are evil creepy shitlords who feel entitled to women's bodies (rape). Think you know a nice guy? They're really a rapist.

Edited to add (after 24 hours): So I didn't anticipate the comments about Red Pill although I suppose I should have. I don't know a lot about the Red Pill stuff, some people like it, some people don't, I don't have any strong feeling either way.

This article isn't intended as a comment on dating advice per se. I am not trying to suggesting what works and what doesn't (although I don't mind comments going in that direction). What I am trying to do is understand what is going on from a feminist perspective when feminists viciously attack "Nice Guys".

The only thing that is new about this article is naming the feminist "advice" to young men as a gender role. So perhaps I should have emphasized that more instead or re-capping all the rest which others have said before. Feminists constantly say they are against gender roles, but that's a very suspect statement. Gender roles are just the things that society tells people they ought to do with the implicit promise that if they conform to society's expectations then they will get some vague goodstuff. In a sense everyone has an idea about how people ought to behave so everyone believes in roles, although I'm not sure MRAs or anti-feminists have any separate role for men and women as feminists do. I guess some do, some don't, but if they do it's not as part of MRA / anti-feminist ideology. Feminists however do have very strong gender roles for men. They are constantly telling men how to behave. and often telling people how to raise their sons. With the "Nice Guy" phenomena we also see that feminists cheat with the gender roles because when young men take on board those roles feminists will tend to viciously attack them anyway.