Green’s Dictionary of Slang

salt n.2

[abbr. colloq. old salt]

as a veteran, an exemplar.

(a) [mid-19C+] a veteran sailor.

(b) [mid-19C] attrib. use of sense 1a.

(c) [mid-19C] in non-naval contexts, a fine example.

(d) [1960s+] a veteran of any experience or discipline.