A sadboi uses his sadness (feigned or actual) as an excuse to behave like a fuckboy. He's like a fuckboy but he covers it up by being "in touch" with his emotions and by being self-deprecating. Claims he's very aware of his flaws and is "working on them," but (and this is key) never actually fixes them.
Girl: Hey, how are you doing?
Sadboi: I know I have a lot of commitment issues and anxieties I'm actively trying to work on and that's why I cant commit to you right now but I would like to continue fucking and doing mundane relationship things for as long as I see convenient.