Green’s Dictionary of Slang

1. a garment that has been stained with grease spots.

1842

Bell’s Penny Dispatch 10 Apr. 3/5: The trousers were the remains of light grey, interspersed with divers spots of grease, giving them the appearance that is significantly termed pepper-and-salt.

2. (US, also pepper-and-salt trousers) striped dress trousers.

1844		‘Jonathan Slick’ High Life in N.Y. I 200: Cousin Beebe come into the room in my old blue coat and pepper and salts.
1887		Richmond Dispatch (VA) 11 Feb. 4/2: He was dressed in [...] pepper-and-salt trousers.
1896		Burlington Wkly Free Press (VT) 28 May 9/3: He wore the pepper-and-salt trousers.
1905		Minneapolis Jrnl 6 May 2/5: [of a jacket] He wore a pepper-and-salt coat and vest and black trousers.
1910		Hawaiian Gaz. (Honolulu) 19 July 6/1: He dusted a spot off the bottom of his brand-new, all-wool pepper-and-salt trousers.
1919		Iron Co. Record (Cedar City, UT) 3 Oct. 7/2: The young man smoothed his new pepper-and-salt trousers with pride.
1920		Cayton’s Wkly (Seattle, WA) 18 Dec. 4/2: The only plan [...] was for Mr Aldrich to go to the theater with his pepper-and-salt trousers combined with the upper half of an evening suit.

3. (US black) black and white people running together in the street, presumably in the civil rights, anti-Vietnam and other demonstrations of the era.

1983

R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].