From Middle English "knak" (a trick or clever device), likely borrowed from Low German or Dutch "knack" (a crack, snap, or trick). By the 16th century it had shifted from meaning a clever trick to meaning an innate ability.

Common in everyday British and American English. Often used in the phrase "get the knack of it," meaning to learn how to do something through practice. Not to be confused with the German "Knackwurst" (a type of sausage) — entirely unrelated despite the same root sound!