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.

Mexicans coming into the U.S.A by swiming

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'.

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

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.