Saying shitty things to people to make them feel bad about themselves so they will lower their standards enough to fuck you. The kind of thing only an incel could come up with.

"You have really pretty eyes for someone with hair like that"

"Wow, your workout routine seems really intense for something who looks like they barely eat"

"I love your makeup! I bet it took you hours to cover what's underneath"

Basically, negging is using backhanded compliments to make someone feel good while also trying to tear them down. You say something nice that makes them feel good, but then say something that makes them feel self-conscious about something else. Very effective narcissistic tactic if used properly which is why the manosphere promotes it. If someone with limited self confidence is met with negging, sometimes they tend to lean into the compliment and use that as a drip of self confidence that they may not get and lean into the person saying it, regardless of the negative addition to it.