Origin and history of wetness

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

Of paint, ink, etc., "not yet dry" from 1510s. In medicine, designating diseases characterized by moist secretions, by 1560s. Suggestive of drinking or drunkenness by 1590s; as opposed to dry in reference to the state-by-state U.S. political battles over permitting sales of liquor, from 1870.

All wet "in the wrong" is recorded from 1923, American English; earlier simply wet "ineffectual" (1896), and perhaps ultimately from slang meaning "drunken" (c. 1700). 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]

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).