The term is a compound word formed within American English in the early 20th century. It originated as a derogatory reference to Mexican migrants who crossed the Rio Grande river, their backs presumably wet from the water, to enter the United States illegally. The word's construction follows a simple descriptive pattern, using two common Germanic-rooted English words to create a vivid, dehumanizing image. Its usage is now widely condemned as a racial and ethnic slur, reflecting a specific historical context of migration and prejudice.

Structure: wet (moistened) + back (rear part of the body) - wet: From Old English wǣt, meaning moist or liquid. In this compound, it functions adjectivally. - back: From Old English bæc, meaning the rear surface of the human body. In this compound, it functions as a noun.