A Dictionary of Music and Musicians/Nut
NUT. 1. Of the Violin (Fr. Sillet; Ger. Sattel). A slip of ebony or ivory (the former chiefly used) glued to the neck of the violin at the upper end of the fingerboard, and over which the strings pass. It is slightly raised above the level of the fingerboard, and serves to keep the strings from touching it except when pressed down by the finger. It existed in the old instruments which preceded the violin, and in them was ruder and larger.
2. Of the Bow (Fr. Hausse; Ger. Frosch). A piece of ebony or ivory, over which the hairs pass, attached to the end of the bow by a metal shank working in a groove cut in the bow. A screw working in the shank serves to tighten or slacken the hairs. The nut is slightly hollowed in the cheeks, and is accurately fitted to the stick by means of a metallic groove. The nut is as old as the bow itself.
The name in both cases is equivalent to 'knob' or 'projection'