Old English wascan "to wash, cleanse, bathe," 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.
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. To wash (one's) hands of something is 1550s, from Pilate in Matthew xxvii.24. To wash up "clean utensils after a meal" is from 1751. Washed up "no longer effective" is 1923, theater slang, from notion of washing up at the end of a job.