"Wetback" is a derogatory term used in the United States for a person of foreign nationality, historically a Mexican citizen, who is an illegal immigrant in the U.S.
Generally used as an ethnic slur, the term was originally coined and applied only to Mexicans who entered Texas by crossing the Rio Grande river, which is located at the Mexican border, presumably by swimming or wading across and getting wet in the process. The non-offensive Spanish term is "mojado" which means "wet". It is often preferred by Mexican-Americans by blood or pure-blood Mexicans who have become U.S. Citizens, to be referred to as "Los Mojados" which translates to "the wet ones" or "wet people".
It also can be used as an adjective and a verb. As an adjective it pertains to illegal Mexican immigrants. It was first used in this way by John Steinbeck in Sweet Thursday as he wrote, "How did he get in the wet-back business?" Its usage as a verb was originally mentioned in 1978 in T. Sanchez's Hollywoodland, where the term meant, "to gain illegal entry into the United States by swimming the Rio Grande".
The first mention of the term in The New York Times is dated June 20, 1920. It was used officially by the US government in 1954, with Operation Wetback, a project which was a mass deportation of illegal Mexican nationals. However, the term was in use in rural parts of the South a century prior to describe escaped African-American slaves by their "wet backs" from swimming to freedom.