Green’s Dictionary of Slang

(a) (also jack salt) a veteran sailor.

1830		N. Ames Mariner’s Sketches 7: The ceremony of shaving on crossing the line was omitted, to the manifest disappointment of the ‘old salts’.
1840		R.H. Dana Two Years before the Mast (1992) 2: My complexion and hands were enough to distinguish me from the regular salt.
1844		C. Selby London By Night I i: I am too old a salt to allow myself to drift on the quicksand of woman’s perfidy.
1861		T. Hughes Tom Brown at Oxford (1880) 74: He can turn his hand to anything, like most old salts.
1870		E. Greey Queen’s Sailors III 250: ‘How are you goin’ to spend your whack, Joseph?’ demanded another old salt.
1885		Bulletin (Sydney) 7 Feb. 13/2: Here was an old salt of some 60 years, nearly 40 of which were spent in coasting.
1885		L. Emanuel Jottings [...] of a Bengal ‘qui hye’ 68: Jack Salt’s three wishes — ‘All the grog in the World!’ — All the baccy in the World!’ [...] ‘Oh, give us a leetle more baccy!’.
1887		W.S. Gilbert ‘The Martinet’ Fifty ‘Bab’ Ballads 214: ‘Down, upstart!’ said the hardy salt.
1895		J.S. Wood Yale Yarns 127: ‘Say, boys— ye must expect a nor’-wester to-night,’ sang out an old salt.
1895		C. Crowe Aus. Sl. Dict. 54: Old Salt, a sailor.
1896		Regiment 4 July 216/1: A mixture of tar-water and the drainings of sugar cargo is not nearly so acceptable even to an old ‘Salt’ as a gill of rum.
1900		Sporting Times 9 June 1/3: [They] asked each individual mariner to leave his pipe at the door. This the salts cheerfully did.
1908		Sun. Times (Perth) 9 Feb. 1/1: Her jealousy of the captain is driving that salt to suicide.
1914		Truth (Melbourne) 31 Jan. 6/1: What a real salt that skipper is the way he brings her alongside the old jetty.
1915		T. McNamara Us Boys 23 Aug. [synd. cartoon strip] ’Lo, old salt. I gotta story I wanna shoot.
1935		E. Anderson Hungry Men 110: Don’t your aunts like old salts?
1949		N. Algren Man with the Golden Arm 195: The Humbodlt Park salt snickered.
1974		(con. 1945) E. Thompson Tattoo (1977) 266: ‘The salt,’ someone crowed.
2017		T. Pluck Bad Boy Boogie [ebook] Vito spun the wheel like an old salt.

(b) attrib. use of sense 1a.

1850

(con. 1843) Melville White-Jacket (1990) 312: It is often observable, that [...] the men who talk the most sailor lingo are the least sailor-like in reality. You may sometimes hear even marines jerk out more salt phrases than the captain of the forecastle himself.

(c) in non-naval contexts, a fine example.

1840

T. Haliburton Clockmaker III 40: Let us walk into half a bushel of these iseters; they are rael salts.

(d) a veteran of any experience or discipline.

1977		(con. 1961) P. Caputo Rumor of War 14: [T]he sight of some old salt with three stripes and a rocker on his sleeves still caused a Pavlovian reaction of terror.
1981		M. Baker Nam (1982) 120: They didn’t want salts from the field to go even near Da Nang.
2003		(con. 1990) A. Swofford Jarhead 60: [P]erhaps the salts’ constant sense of impending doom had rung loudly enough that they listened [...] The salts were tired and mean and the last thing they needed was a war.