Daniel Cassidy, in what is certainly one of the worst books ever written, How The Irish Invented Slang, claims that the English word knick-knack comes from Irish. It doesn’t. It is derived from the English knack, which now means a trick but formerly meant a trinket or small object. John Heywood used the word knack in 1540: “Needles, thread, thimble, shears, and all such knacks.” Shakespeare also used it in The Taming of the Shrew in 1596. (Cassidy claims that knack comes from Irish gnách but gnách doesn’t mean a trick or special skill and it only marginally has the sense of custom or habit – gnás or nós would be much more common in this sense.) By 1618, John Fletcher was talking about knick-knacks as tricks: “If you use these knick-knacks, This fast and loose.” But by the end of the 17th century, a knick-knack was exclusively used of a trinket. In Scotland, the word became nig-ma-nag.
Cassidy disagrees that knick-knacks is a rhyming jingle based on the word knack. To him, knick-knacks come from the Irish word neamhghnách meaning unusual. There are many reasons for objecting to this. Firstly, neamhghnách is an adjective and cannot be used as a noun. Secondly, the sound is very unlike the English word knick-knack. And thirdly, why wouldn’t an Irish speaker use one of the many words which really mean knick-knacks in Irish, like giuirléidí, mangaisíní, áilleagáin, deasagáin or gréibhlí, rather than misusing an adjective? They wouldn’t, of course. It’s just more old knick-knack paddywhackery from this boring, childish idiot.