What does "knock out" mean?

Also, knock out cold. Render unconscious by a blow or some other means. For example, It was just a swinging door, but it knocked her out, or Just one of those sleeping pills can knock you out cold. [Late 1500s]
Make tired, exhaust, as in That sightseeing tour knocked me out.
Render useless or inoperative, as in The storm knocked out the power.
See knock off, def. 2.
See knock oneself out.

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