The Etymology of Knack
Knack belongs to a quiet family of English imitative words — knock, knack, click, clack, clink — that started life as sound effects and graduated into nouns and verbs. The earliest recorded sense in the 14th century was a sharp blow or sharp noise, which is what the word still sounds like; from there it slid by metonymy into the cleverness associated with such precision: a knack became a small ingenious device or trick, the kind of thing a craftsman might pull off with a deft tap. By 1500 the meaning had abstracted again, from device to skill — to have a knack for something is to do it neatly, easily, almost effortlessly. The reduplicated knick-knack appears around 1610 for a small clever ornament or toy, and by the 19th century specifically for the small decorative trinkets cluttering Victorian mantelpieces. Knack and knack-kneed (knock-kneed) share no etymology — the latter is just knock with an unstable spelling.