"to strike with a sharp sound," late 15c., echoic. Earlier (c. 1400) as a noun meaning "abrupt stroke." Especially "to chip or break by a sharp blow" (1530s), the sense shifting from the sound to the act that makes it. Especially of the method of sharpening flints from 1862. Related: Knapped; knapper; knapping. For pronunciation, see kn-.