To strike or bump (someone or something) out.
I accidentally knocked out the glass in my picture frame.
To render unconscious, as by a blow to the head.
The boxer knocked out his opponent in the third round.
To put to sleep.
The allergy pill knocked him out for a good three hours.
To exhaust.
Running errands all day really knocked him out.
To complete, especially in haste; knock off.
They knocked out the entire project in one night.
To cause a mechanism to become non-functional by damaging or destroying it.
The antitank gun knocked out the enemy tank.
To eliminate.
To communicate (a message) by knocking.
The prisoner knocked out a message on the wall for the prisoner in the adjoining cell.
To lose the scent of hounds in fox-hunting.
(Oxford University slang) To leave college after hours—after half-past ten at night when the doors has been locked.