wetback n.

Old English wæt "moist, rainy, liquid," also as a noun, "moisture; liquid drink," from Proto-Germanic *wed- (source also of Old Frisian wet). Also in part from cognate Old Norse vatr, and all ultimately from PIE root *wed- (1) "water; wet."

Of paint, ink, etc., "not yet dry" from 1510s. Opposed to dry in reference to the U.S. battles over prohibition from 1870. Wet blanket "person who has a dispiriting effect" is by 1830, from the notion of a person who throws a damper on social situations in the way a wet blanket smothers a fire (the phrase is attested in this literal sense from 1660s).

Do we not know them, those wet blankets who come down on our pleasant little fires and extinguish them, with no more ruth than the rain feels when it pours on the encampment of the merry picnic party, or floods the tents of a flower show? ["Wet Blankets," in Godey's Lady's Book and Magazine, February, 1871]

All wet "in the wrong" is recorded from 1923, American English; earlier simply wet "ineffectual," and perhaps ultimately from slang meaning "drunken" (c. 1700). Wet-nurse is from 1610s. The diver's wet-suit is from 1955. Wet dream is from 1851; in the same sense Middle English had ludificacioun "an erotic dream."

He knew som tyme a man of religion, þat gaff hym gretelie vnto chastitie bothe of his harte & of his body noghtwithstondyng he was tempid with grete ludificacions on þe nyght. ["Alphabet of Tales," c. 1450]

Old English bæc "back," from Proto-Germanic *bakam (cognates: Old Saxon and Middle Dutch bak, Old Frisian bek), with no known connections outside Germanic. In other modern Germanic languages the cognates mostly have been ousted in this sense by words akin to Modern English ridge (such as Danish ryg, German Rücken).

Many Indo-European languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. In other cases, a modern word for "back" may come from a word related to "spine" (Italian schiena, Russian spina) or "shoulder, shoulder blade" (Spanish espalda, Polish plecy).

By synecdoche, "the whole body," especially with reference to clothing. The meaning "upright part of a chair" is from 1520s. As a U.S. football position by 1876, so called from being behind the line of rushers; further distinguished according to relative position as quarterback, halfback, fullback. To turn (one's) back on (someone or something) "ignore" is from early 14c.

To know (something) like the back of one's hand, implying familiarity, is first attested 1893 in a dismissive speech made to a character in Robert Louis Stevenson's "Catriona":

If I durst speak to herself, you may be certain I would never dream of trusting it to you; because I know you like the back of my hand, and all your blustering talk is that much wind to me.

The story, a sequel to "Kidnapped," has a Scottish setting and context, and the back of my hand to you was noted in the late 19th century as a Scottish expression meaning "I will have nothing to do with you" [see Longmuir's edition of Jamieson's Scottish dictionary]. In English generally, the back of (one's) hand has been used to imply contempt and rejection at least since 1300. Perhaps the connection of a menacing dismissal is what made Stevenson choose that particular anatomical reference.