also knock-out, in fighting, 1887, from verbal phrase knock out "to stun by a blow for a 10-count" in boxing, short for to knock out of time; see knock (v.) + out (adv.). Slang meaning "excellent thing or person" is from 1892; specifically "beautiful woman" by 1953. As an adjective from 1896 in the tournament competition sense, 1898 in fighting. To knock oneself out "make a great effort" is from 1936.
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Harper, D. (n.d.). Etymology of kayo. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/kayo