|Noah Webster's Dictionary|
(n. pl.) Two small, concave shells of ivory or hard wood, shaped like spoons, fastened to the thumb, and beaten together with the middle finger; -- used by the Spaniards and Moors as an accompaniment to their dance and guitars.
Int. Standard Bible Encyclopedia
kas'-ta-nets, kas-ta-nets' (mena`an`im): Are mentioned in 2 Samuel 6:5 among the musical instruments upon which David and the house of Israel played at the time of the bringing up of the ark out of the house of Abinadab. This word is incorrectly translated "cornets" in the King James Version. The castanet was probably about the same kind of instrument as the Egyptian sistrum, and the Revised Version (British and American) has "sistra" in the margin of 2 Samuel 6:5. The sialrum was a loop-shaped metal frame through which were passed loose rods at the ends of which were rings. The instrument was held by a long handle and was rattled during songs and dances. It was used in Egypt in religious worship or to scare away evil influences. There is only the one reference to this instrument in the Bible.
A. W. Fortune
Castanets (1 Occurrence)
2 Samuel 6:5 David and all the house of Israel played before Yahweh with all manner of instruments made of fir wood, and with harps, and with stringed instruments, and with tambourines, and with castanets, and with cymbals. (WEB ASV NAS RSV)