knack (n.)

"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-.

also knickknack, nicknack, "a pleasing trifle, toy," 1570s, a reduplication of knack (n.) "ingenious device, toy, trinket" (1530s), a specialized sense of knack (n.) "stratagem, trick."

Middle English spelling of a common Germanic consonant-cluster (in Old English it was graphed as cn-; see K). The sound it represented persists in most of the sister languages, but in English it was reduced to "n-" in standard pronunciation by 1750, after about a century of weakening and fading. It was fully voiced in Old and Middle English.