Wetback Towel May 2006, Talavera de la Reina 100 x 160 cm Embroidery on cotton towel i Wetback is a derogatory term for a Mexican who illegally enters the United States of America from its southern border, often by either swimming or wading across the Rio Grande. Wet back (Spanish: espalda mojada) is also used in an equally pejorative way in Spain to refer to illegal immigrants from Morocco who cross the Straits of Gibraltar.