Wetback is a derogatory term for a person who has immigrated illegally to the United States. Commonly referring to Mexicans who have crossed the border illegally, the term originated with those who entered Texas by crossing the Rio Grande, presumably by swimming or wading across and getting his or her back wet in the process.
The first mention of the term in the New York Times is dated June 20, 1920. It was used officially by the U.S. Government in 1954, with Operation Wetback, a project where a large number of Mexican nationals were deported.
Wetback became a popular slur in other states with large Mexican populations, as well, such as California, Arizona and New Mexico, especially among those with Anti-Mexican sentiment who found it a preferable way of defining them. The slur is even more insulting when referring to all Mexican people, rather legal or illegal, even Mexican Americans who are born in the U.S.
An equivalent Spanish term, mojado, is sometimes used by legal immigrants and native-born Hispanic and Latino Americans as an insult to illegal immigrants. Another equivalent Spanish term for an illegal immigrant who crosses the border into the U.S. from Mexico is alambrista.
Racial profiling of "wetbacks" extended to others of Latin American descent, as well, especially those with the physical distinctiveness of mestizos. To some Hispanic/Latino people, the use of the term is just as offensive as wop, chink, or nigger.