knockout

Knock out as a verb — to render unconscious by a blow — appears in boxing contexts by the late 18th century. The noun 'knockout' appears by the 1860s. The compound is simple: knock (from Old Norse knoka, to beat or strike) plus out (to the exterior, beyond consciousness). A knockout was a blow that sent a boxer beyond the boundary of consciousness — the out of the ring, the out of awareness.

The Queensberry Rules (1867), drafted by John Graham Chambers and endorsed by John Sholto Douglas, 9th Marquess of Queensberry, codified the knockout in boxing: a fighter who could not rise within ten seconds of being knocked down was declared knocked out, losing the bout. These rules also required gloves, three-minute rounds, and banned wrestling. The Queensberry Rules transformed boxing from a brutal endurance contest into a sport with defined victory conditions.

The knockout tournament format — where each loss is elimination — was used in sport long before the boxing sense. Cricket tournaments, tennis tournaments, chess tournaments have used single-elimination brackets since at least the 18th century. But 'knockout' as the label for the format borrowed the boxing sense: you are hit and you are out. The metaphor of the blow became the metaphor of the bracket.

In British usage, 'a knockout' also became high praise — a knockout is something superlatively impressive. 'She was a knockout' meant beautiful enough to be stunning. The boxing blow that eliminates opposition became the compliment for anything that overwhelms. The physical impact translated into aesthetic impact.