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. Slang meaning "attractive person" is from 1892. To knock oneself out "make a great effort" is from 1936.
实用例句
1. He delivered a knockout blow to all of his rivals.