From Middle Dutch or Middle Low German *knack*, meaning a sharp crack or snap — originally suggesting the precise, practiced click of a craftsman's tool hitting its mark. By the 17th century it shifted in English to mean a clever trick, then a natural aptitude.

Common in both British and American English, "knack" is often used conversationally to compliment someone's instinctive ability — e.g., "He's got the knack." It's warm and informal in tone without being slangy. Note: avoid confusing with the British slang "knackered" (exhausted), which is etymologically unrelated.