1. To destroy or break forcefully
2. To render unconscious
3. To eliminate
4. To empty (as of tobacco) by knocking out
5. To overwhelm with admiration

1. When you knock people out, you hit them hard enough to cause them to lose consciousness. When people are knocked out, they are hit by someone or something hard enough to cause them to lose consciousness.
2. If something knocks you out, it impresses or surprises you a lot.
3. When you try very hard to please other people, you knock yourself out or knock yourself out to do something.
4. When soldiers knock out a piece of the enemy's equipment, they destroy it or damage it enough so that it no longer operates.