"attack, aggression, assault," 1620s, anslaight, apparently somehow from or on analogy of Dutch aanslag "attack," from Middle Dutch aenslach, from aen "on" (see on) + slach "blow," related to slaen "slay." Early sources say the word was from Dutch, but the forms do not correspond well. The spelling would have been influenced by English slaught (n.) "slaughter," from Old English sleaht (see slaughter (n.)), but this word, obsolete since c. 1400, can't be the source of the modern one unless the record is imperfect. There is no record of onslaught in 18c.; apparently the word was revived by Scott.