Learn more about Rabbah from the Easton’s Bible Dictionary

Or Rab'bath, great. (1.) "Rabbath of the children of Ammon," the chief city of the Ammonites, among the eastern hills, some 20 miles east of the Jordan, on the southern of the two streams which united with the Jabbok. Here the bedstead of Og was preserved (Deut. 3:11), perhaps as a trophy of some victory gained by the Ammonites over the king of Bashan. After David had subdued all their allies in a great war, he sent Joab with a strong force to take their city. For two years it held out against its assailants. It was while his army was engaged in this protracted siege that David was guilty of that deed of shame which left a blot on his character and cast a gloom over the rest of his life. At length, having taken the "royal city" (or the "city of waters," 2 Sam. 12:27, i.e., the lower city on the river, as distinguished from the citadel), Joab sent for David to direct the final assault (11:1; 12:26-31). The city was given up to plunder, and the people were ruthlessly put to death, and "thus did he with all the cities of the children of Ammon." The destruction of Rabbath was the last of David's conquests. His kingdom now reached its farthest limits (2 Sam. 8:1-15; 1 Chr. 18:1-15). The capture of this city is referred to by Amos (1:14), Jeremiah (49:2, 3), and Ezekiel (21:20; 25:5).
(2.) A city in the hill country of Judah (Josh. 15:60), possibly the ruin Rubba, six miles north-east of Beit-Jibrin.