Old English wæt "moist, liquid," from Proto-Germanic *wætaz (cf. Old Frisian wet ). Also from the Old Norse form, vatr. All related to water (n.1).
Wet blanket "person who has a dispiriting effect" is recorded from 1879, from use of blankets drenched in water to smother fires (the phrase is attested in this literal sense from 1660s). 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. 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 wætan "to be wet;" see wet (adj.). Related: Wetted; wetting.
Immature, inexperienced, and naïve: “Don't rely on his advice about girlfriends; he's still wet behind the ears.”