1580s, "act of lying concealed for the purpose of attacking by surprise," essentially a variant form of ambush (q.v.), "now more formal as a military term" [OED]. It is a reborrowing of that French word after it had been Italianized: Ambuscade is from French embuscade (16c.), Gallicized from Italian imboscata, literally "a hiding in the bush," compounded from the same elements as Old French embuscher.
Sometimes in early use ambuscado, with a faux Spanish ending of the sort popular in 17c. As a verb, "attack from a concealed position," 1590s.