/ næk; NAmE / noun [ sing. ]
1.
knack (of / for sth) | knack (of / for doing sth) a special skill or ability that you have naturally or can learn :
It's easy, once you've got the knack.
He's got a real knack for making money.
2.
knack of doing sth ( BrE ) a habit of doing sth :
She has the unfortunate knack of always saying the wrong thing.
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WORD ORIGIN
late Middle English (originally denoting a clever or deceitful trick): probably related to obsolete knack sharp blow or sound , of imitative origin (compare with Dutch knak crack, snap).