Origin and history of outwash

"apply water or liquid to for purposes of cleansing," Middle English washen, from Old English wascan "cleanse, bathe," also "cleanse spiritually" (of sin, guilt); transitive sense in late Old English; from Proto-Germanic *watskan "to wash" (source also of Old Norse vaska, Middle Dutch wasscen, Dutch wassen, German waschen), from PIE root *wed- (1) "water; wet." Related: Washed; washing.

In the literal sense used mainly of clothes in Old English (the principal verb for washing the body, dishes, etc. being þwean). Old French gaschier "to stain, soil; soak, wash" (Modern French gâcher) is from Frankish *waskan, from the same Germanic source. Italian guazzare also is a Germanic loan-word. See gu-.

By 1530s as "rinse the mouth." To wash up "clean table utensils after a meal" is from 1751 (compare washed-up). To wash down (solid food, with a liquid) is by c. 1600. To wash (one's) hands of "forsake involvement in" (an iniquity, etc.) is attested by 1550s (Lady Jane Grey), an image from Pilate in Matthew xxvii.24.

in Old English a common prefix with nouns, adjectives, adverbs, and verbs, "out, outward, outer; forth, away," from out (adv.). The use was even more common in Middle English, and also with the senses "outer, outside, on the outside, from without, external, externally; apart; greatly, extremely; completely, thoroughly, to completion." Other senses of out that extended into the use as a prefix include "beyond the surface or limits; to the utmost degree; to an explicit resolution."

In composition out has either its ordinary adverbial sense, as in outcast, outcome, outlook, etc., or a prepositional force, as in outdoors, or forms transitive verbs denoting a going beyond or surpassing of the object of the verb, in doing the act expressed by the word to which it is prefixed, as in outrun, outshine, outvenom, etc. In the last use especially out may be used with almost any noun or verb. [Century Dictionary]