Green’s Dictionary of Slang

wetback n.

also wet
[the condition of the immigrants who trad. swim the Rio Grande as the best means of beating border checks. Despite its reputation as a racist slur, Hispanics accept it; thus Mex.Am. self-description los mojados, the wet ones]

[1920s+] a derog. term for an illegal Mexican immigrant to the USA; thus for Mexicans and so Hispanics in general.