onslaught n.

"in a position above and in contact with; in such a position as to be supported by;" also noting the goal to which some action is or has been directed; "about, concerning, regarding; in a position to cover;" as an adverb, "in or into a position in contact with and supported by the top or upper part of something; in or into place; in place for use or action; into movement or action; in operation," Old English on, unstressed variant of an "in, on, into," from Proto-Germanic *ana "on" (source also of Dutch aan, German an, Gothic ana "on, upon"), from PIE root *an- (1) "on" (source also of Avestan ana "on," Greek ana "on, upon," Latin an-, Old Church Slavonic na, Lithuanian nuo "down from").

Also used in Old English in many places where we now would use in. From 16c.-18c. (and still in northern England dialect) often reduced to o'. Phrase on to "aware" is from 1877.

c. 1300, "the killing of a person, murder; the killing of large numbers of persons in battle;" mid-14c., "the killing of cattle or sheep or other animals for food;" from a Scandinavian source akin to Old Norse slatr "a butchering, butcher meat," slatra "to slaughter," slattr "a mowing," related to Old Norse sla "to strike," from Proto-Germanic *slagan- (see slay (v.)).

The form was perhaps influenced by obsolete slaught "killing, manslaughter, carnage; butchery of animals," the native cognate, which is from Old English sliht, sleht, slieht "stroke, slaughter, murder, death; animals for slaughter;" as in sliehtswyn "pig for killing." The Elizabethans had an adjective slaughterous.