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Generally used as an ethnic slur, the term was originally coined and applied only to Mexicans who entered the U.S. state of Texas from Mexico by crossing the Rio Grande, which forms the border between Texas and Mexico, presumably by swimming or wading across the river and getting wet in the process.
A similar term,"Los Mojados", which translates to "wet people", has become obsolete.
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 immigrants.
The term can also be used as an adjective or a verb. As an adjective, it pertains to activities involving illegal Mexican immigrants. The earliest known recorded use in this way is by John Steinbeck in Sweet Thursday as he wrote, "How did he get in the wet-back business?" It was originally used as a verb in 1978 in Thomas Sanchez's Hollywoodland with the meaning, "to gain illegal entry into the United States by swimming the Rio Grande".