To the chief Musician on Neginoth upon Sheminith, a Psalm of David. What is designed by "the chief musician", and what is meant by "neginoth", have been observed preciously, See Gill on "Ps 4:1". As for "sheminith", it seems to be one particular sort of the "neginoth", or stringed instrument, which this man had the care of; see the title of Ps 12:1: which most of the Hebrew writers understand of the harp of eight strings, to which this psalm was set; and which, from the number of its strings, was called "sheminith", which signifies "the eighth": and this receives confirmation from 1Ch 15:21.
The eighth string was added to the harp by Simonides, according to Pliny : but if such an harp is here meant, this refutes it, for David lived long before Simonides. Though some have thought it refers to a poem or song of eight notes, to the tune of which this psalm was sung; or to the eighth note, which was grave, and which we call the bass.
As for the eighth day of circumcision, of which some Jewish writers mystically interpret it; or the eighth, that is, the first day, or Lord's day, to which some of the ancient Christian writers refer it, or the eighth age, or millennium, as Theodoret; these can by no means be admitted of.
The occasion of it was either some bodily disease the psalmist laboured under, or some distress of soul, on account of sin; and the rather this seems to be the case, seeing the psalm begins with the same words as Ps 38:1, in which the psalmist so much bewails his iniquity.
Some have thought it was drawn up for the use of any and every sick person; and others say it was written on the account of Israel in captivity, who were then as sick persons : but rather the occasion of it was bodily sickness, inward guilt of conscience for sin, and distress by enemies; as appears from, Ps 6:3.
Targum, Jarchi, Kimchi, & Ben Melech in loc. Nat. Hist. l. 7. c. 56. In Aben Ezra in loc. Vid. Kimchi & Aben Ezra in loc.Ver. 1.