E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Wolf (in music).
In almost all stringed instruments (as the violin, organ, piano, harp, etc.) there is one note that is not true, generally in the bass string. This false note is by musicians called a wolf.
The squeak made in reed instruments by unskilful players is termed a goose.
Nature hath implanted so inveterate a hatred atweene the wolfe and the sheepe, that, being dead, yet in the operation of Nature appeareth there a sufficient trial of their discording nature; so that the enmity between them seemeth not to dye with their bodies; for if there be put upon a harpe strings made of the intralles of a sheepe, and amongst them one made of the intralles of a wolfe the musician cannot reconcile them to a unity and concord of sounds, so discording is that string of the wolfe.Ferne: Blazon of Gentrie (1586).
Here Mr. Ferne attributes the musical wolf to a wolf-gut string; but the real cause is a faulty interval. Thus, the interval between the fourth and fifth of the major scale contains nine commas, but that between the fifth and the sixth only eight. Tuners generally distribute the defects, but some musicians prefer to throw the whole onus on the wolf keys.