'Salty' means bitter, resentful or upset, especially in a petty way — often after losing or being publicly embarrassed. 'He's salty about losing' = 'he's bitter about losing'. Originally African-American Vernacular English and gaming slang. If your teen calls someone salty, they are saying the person is overreacting to a small defeat. Harmless observational vocabulary; only worth a conversation if it gets used dismissively in real arguments.