Originally a nick name for Mexicans greaser, beaner, pepper belly, either legally or illegally in the country. Originated in the American Southwest border states, Texas, New Mexico, Arizona, and California. Through common usage has come to mean citizens of any nation who have immigrated here illegally.

"Wetback is a derogatory term used to describe Mexicans who have immigrated illegally to the United States by swimming or wading across the Rio Grande--the river that separates the U.S. from Mexico. U.S. Border Patrol began using the word in 1944 to refer to illegal Mexican immigrants who were easily identifiable by their wet clothing.
In 1954, the U.S. Immigration and Naturalization Service made the word "wetback" official by naming the mission to remove illegal immigrants from the United States, Operation Wetback . In response to the increase of immigrants during the early 1950s, the agency developed the program to force immigrants (particularly Mexicans) back to their home countries. Aproximately one million Mexican Americans were deported in one year.

Today the term "wetback" is often used to express animosity towards Central American or Latin American immigrants--legal and illegal--who do not speak English. Meanwhile, in an attempt to reclaim the word, some Mexican-Americans call themselves Los Mojados, meaning "the wet ones."

~ This definition courtsey of www.gurl.com

Mexicans coming into the U.S.A by swiming

Illegal immigrants from Mexico who try to swim/ferry through the river illegally. They're the ones that make all hispanics look like idiots.

A derogitory term used to describe illegal immigrants from Mexico. This originates from them jumping the boat and swimming to shore (undetected) hence having 'wet backs'.

Another name for Latin Americans living in the United States.

Illegal aliens from Mexico.
As the land near the USA-Mexico border is mainly very hot desert, the people who try to illegal cross the border on foot often become very sweaty. As a result, their backs are wet with sweat and they are known by this feature.