1. Pre-2000s, the word is used to describe a person (male) who is generally social and positive to friends, family, and anybody he associates. He is dependable, means well, and looks out for people he's close to.

In the dating pool. Some guys discovered they can come on too hard and be rejected, so they decided that if they become 'nice' and act friendly to the girl, she would grow to trust him and later allow him to take her. Even if she wasn't looking for a relationship or already has someone. Unfortunately, once he realises, the guy becomes resentful and views her as a slut for sleeping with one person she's committed to, or stupid enough to date a bad boy (note that any male that already claims his girl, regardless of look, criminal history (existing or nonexisting), or personality, will be the 'asshole').

Thus, they are people who always whine to themselves (at forums, to each other, or a hapless victim) wondering why, as they're such a 'Nice Guy'. It's become a word that's taken an ironic meaning.

2. An annoying self-designated third-wheeler who puts an unhealthy interest in a couple's relationship, but won't talk to or socialise with girl's own boyfriend. But will completely slam her to friends the minute he realises that she won't cheat/dump boyfriend, let alone do the 'pity-fuck' for him regardless if she feels chemically attracted to him or not. Thus causing a whole line of insults to be written about her online.

A man who thinks that basic respect for women is a personality trait and wears said "personality trait" like a trophy; an incel.

The light version of an incel

Underhated guy

A charming, phony, superficial guy who is scared of girls, especially the ones with pigtails.

A man who thinks he deserves a relationship/sex because he’s nice to women.