knack (n.)

mid-14c., "a deception, trick, device," a word of uncertain origin. Perhaps from or related to a Low German word meaning "a sharp sounding blow" (compare Middle English knak, late 14c.; German knacken "to crack;" also knap) and of imitative origin. Sense of "special skill" (in some specified activity) is first recorded 1580s, if this is in fact the same word. In old slang (mid-18c. to mid-19c.) nacky meant "full of knacks; ingenious, dexterous." For pronunciation, see kn-.