Green’s Dictionary of Slang

a derog. term for an illegal Mexican immigrant to the USA; thus for Mexicans and so Hispanics in general.

1929		Foreign Affairs Oct. 101: The peon walks or swims across [...] and is welcomed by his countrymen here as a ‘wet back’ .
1948		W. Burroughs letter 2 Dec. in Harris (1993) 29: The broker backs a truck up to the Rio Grande and loads it with Mexican ‘Wetbacks.’.
1952		Lait & Mortimer USA Confidential 140: The truth is, Mexican peasants are so anxious to work for the acknowledged low wages, they swim across the rivers at night to enter the U.S. That is why they are called wet-backs.
1960		C. Cooper Jr Scene (1996) 93: Sometimes a Chink or wetback gets into the city with some [i.e. cocaine]: it doesn’t last long.
1975		J. Wambaugh Choirboys (1976) 83: You write half the time like some ignorant wetback.
1979		G. Swarthout Skeletons 157: They picked up a skilled wet and pumped him dry.
1981		J. Ellroy Brown’s Requiem 18: He hires nothing but wetbacks, and pays them the minimum wage.
1990		J. Ellroy ‘Gravy Train’ in Pronzini & Adrian Hard-Boiled (1995) 490: Wetbacks hot to bring the family up from Calexico.
1999		E. Bunker Mr Blue 114: Outside, around the corner, he had two ‘wetbacks’ from Mexico.
2008		T. Piccirilli Fever Kill 77: Big thing now is wetbacks coming up from south of the border.
2017		S.M. Jones August Snow [ebook] Even if you’d been an American citizen for twenty, thirty, a hundred years, you were still a ‘wet’.
2020		D. Winslow ‘The Last Ride’ in Broken 286: [H]is daddy used to cuss out the ‘wetbacks’.