1920, Temporary admission of illiterate Mexican laborers: Hearings before the Committee on Immigration and Naturalization, House of Representatives, 66th Congress, 2nd Session, p. 141, Mr. HUDSPETH. […] I stood here and did all in my power to keep the “wetback” […] from coming across. And I stand here to-day asking for that. The CHAIRMAN. […] What is a “wetback”? Mr. HUDSPETH. You have heard a “wetback” described; he is a fellow that crosses the river surreptitiously.
Source: wiktionary