What's the meaning of the phrase 'Knick-knack'?
The expression 'knick-knack' is commonly used to refer to a dainty little trinket or ornament. Previously, it also meant 'a neat trick or sleight of hand'.
What's the origin of the phrase 'Knick-knack'?
Knick doesn't mean anything in itself in this term; it is merely a reduplication of knack.
As 'knick-knick' originates from 'knack' it isn't surprising that the two different meanings of 'knick-knack' derive from the two different meanings of knack.
Knack = 'A dexterous facility or trick.'
When knack was first used it meant 'an underhand trick; an adroit or ingenious method of doing something.
Geoffrey Chaucer printed the Middle English spelling of the word in The Book of the Duchesse, circa 1369:
She ne used no suche knakkes smale.
That was the meaning of knack that was extended to 'knick-knack' by John Fletcher in his work The Loyall Subject, circa 1625:
If you use these knick-knacks, This fast and loose.
It is likely that this version of 'knick-knack' ultimately derived from an earlier term crinkum-crankum.
Knack = 'An ingenious contrivance; a toy, a trinket.'
John Heywood, used the second meaning of the word knack in his piece titled The Playe called The Foure PP., 1540:
"Needles, thread, thimble, shears, and all such knacks."
Shakespeare also used it in The Taming of the Shrew, 1596:
"Why 'tis a cockle or a walnut-shell, A knacke, a toy, a tricke, a babies cap."
By 1682, the dexterous trick meaning was rarely used. A translation of Nicolas Boileau-Despréaux's Le Lutrin used the term with the meaning we currently have for it, that is, small trincket:
"Miss won't come in to Buy, before She spies the Knick-knacks at the Dore."
Knack = 'A special talent or easy facility.'
Although the 'adroit method of doing something' was less used after the 16th century, it didn't entirely die out. In the 20th century it re-emerged as the name for 'a special easy facility'. So, someone who can do something that others find difficult but without any apparent effort might be said to have the knack of doing it.
That was the meaning in the title of the 1965 comic film The Knack... and How To Get It. The film focussed on the lead character's attempt to get 'the knack', on that case, the ability to seduce women.