Knack, n.
1. A petty contrivance; a toy; a plaything; a knickknack. A knack, a toy, a trick, a baby's cap. Shak.
2. A readiness in performance; aptness at doing something; skill; facility; dexterity. The fellow . . . has not the knack with his shears. B. Jonson. The dean was famous in his time, And had a kind of knack at rhyme. Swift.
3. Something performed, or to be done, requiring aptness and dexterity; a trick; a device. "The knacks of japers." Chaucer. For how should equal colors do the knack ! Pope.
1 other Webster entry under this spelling
Knack, v. i.
1. To crack; to make a sharp, abrupt noise to chink. [Obs. or Prov. Eng.] Bp. Hall.
2. To speak affectedly. [Prov. Eng.] Halliwell.
Shades of meaning
Fernald has no entry of its own for knack; he lists it under ease.
He lists it without comment; the entries above explain its neighbors.