1.  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]
2.  Make tired, exhaust, as in That sightseeing tour knocked me out.
3.  Render useless or inoperative, as in The storm knocked out the power.
4.  See knock off, def. 2.
5.  See knock oneself out.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.