Easton's Bible Dictionary (1897)
Eight; octave, a musical term, supposed to denote the lowest note sung by men’s voices (1 Chr. 15:21; Ps. 6; 12, title).
Smith's Bible Dictionary (1863)
(eighth), a musical term found in the title of (Psalms 6:1) A similar direction is found in the title of (Psalms 12:1) Comp. 1Chr 15:21 It seems most probable that Sheminith denotes a certain air known as the eighth, or a certain key in which the psalm was to be sung.
Hitchcock's Bible Names (1869)
eighth (an eight-stringed instrument)