Green’s Dictionary of Slang

experienced, esp. after overcoming some form of problem.

1889		F. Oates Matabele-Land 236: The old man tells me that a man gets a pain in his head and lies down, and next morning, if he is alive, he is ‘salted’ .
1892		Stevenson & Osbourne Wrecker 9: Mr. London Dodd, though he was new to the group of the Marquesas, was already an old salted trader; he knew the ships and the captains .