Easton's Bible Dictionary

Dumah - silence, (comp. Ps. 94:17), the fourth son of Ishmael; also the tribe descended from him; and hence also the region in Arabia which they inhabited (Gen. 25:14; 1 Chr. 1:30).

There was also a town of this name in Judah (Josh. 15:52), which has been identified with ed-Domeh, about 10 miles southwest of Hebron. The place mentioned in the "burden" of the prophet Isaiah (21:11) is Edom or Idumea.