Webster's Revised Unabridged Dictionary
Cf. Prov. G. knagge, a knot in wood, Sw. knagg, Dan. knag, a hook to hand clothes on, a bracket; Gael. & Ir. cnag, peg, knob
Chambers's Twentieth Century Dictionary
From a root found in Ir. and Gael. cnag, a knob; cf. Dan. knag, Ger. knagge.
You'll not forget my directions, Miss Knag?
"The Life And Adventures Of Nicholas Nickleby" by Charles Dickens
Then you may let it hang on the knag where it is all day.
"The Shepherd's Calendar" by James Hogg