1. To be or be rendered unconscious, as by a physical blow, medication, or other means. I had a shot at a championship boxing title, but I was knocked out cold in the semi-finals. Jim's been knocked out cold by that painkiller for a few hours now. Sam drank a bit too much on her 21st birthday and was knocked out cold by 10 PM.

2. To be in a deep, soundless sleep, especially due to exhaustion or fatigue. The kids are all upstairs, knocked out cold from our day's hike. My girlfriend and I were knocked out cold as soon as we got home from our four-hour cycle around the city.