Salty

A person who is jealous and resentful either because of a loss or because someone else got something they didn't.

It can be used seriously, but usually it's in a joking manner.

"He's salty because he didn't get a phone when he was her age."

"They're salty 'cause that girl stole the soccer ball right from underneath them when they were about to score."

"She's so salty because she lost a game of Mario Kart."

"I'm still salty, okay?"