Boyd’s Bible Dictionary:
(keep holiday, sacrifice). A stringed instrument like the psaltery (Amos 6:5). [PSALTERY.]
Concise Bible Dictionary:
A stringed instrument. The word is nebel, and is often translated “PSALTERY.” Its exact form is not known (Isa. 5:12; Isa. 14:11; Amos 5:23; Amos 6:5).