an individual who enters this country illegally across the Rio Grande--" "And you entered this country illegally across the Atlantic Ocean

Wetback is a derogatory term for a Mexican or Central American who is in the United States illegally. The term is based on the presumption that the person in question swam or waded across the Rio Grande to get into the United States, getting their back wet in the process. It may also refer to the shirt of a Mexican laborer becoming darkened with sweat, or the accumulation of sweat in general on a Mexican's back, resulting from labor-intensive, outdoors work.