Surprisingly calm, cool, and collective on a certain situation or scenario, especially one that has been dealt with several times before. (financial issues, haters, etc.)
"Seasoned" is the opposite of "salty."
The term comes from "seasoning", a form of condiment similarly used to salt, the origin word for "salty".
"All these haters salty; I'm too seasoned, homie."
"HOW THE HELL CAN YOU BE CHILL IN A SITUATION LIKE THIS?!"
"Rich, he's too seasoned to panic."