Sudden Onset Confidence Syndrome 

When someone, through some means (usually weight loss), gains a large and sudden boost to their confidence that turns them into egotistical dickheads. It can also exacerbate egos for people who were faking it and now that they have real confidence it makes them appear like they're bragging all the time.

Fella: Hey, did you see that Gary lost a ton of weight recently?
Mate: Yeah! Notice how he hates fat people now? And he scoffs at chicks that he previously would have crawled dick first through a valley of glass just for them to notice him?
Fella: I have!

Mate: Ya boy's got a bad case of Sudden Onset Confidence Syndrome.

Sudden Onset Confidence Syndrome by RussNuts July 16, 2019