A mexican with no green card, too lazy to get one; therefore swims to the U.S thus becoming a "wet-back".
The Wet-backs pick srawberries, work our fast foods, and sell flowers, oranges, cherries etc. on main streets.
A derogatory word used to describe Mexicans who illegally enter the United States through the Texas portion of the Rio Grande. By going under the border fence but over the river they would wet their backs.
That guy is soaking wet and looking a little out of place must be a Wet-Back.