Jamieson, Fausset and Brown on Numbers 21:2-3

2-3. Israel vowed a vow unto the Lord--Made to feel their own weakness, they implored the aid of Heaven, and, in anticipation of it, devoted the cities of this king to future destruction. The nature and consequence of such anathemas are described (Le 27:1-34; De 13:1-18). This vow of extermination against Arad [Nu 21:2] gave name to the place Hormah (slaughter and destruction) though it was not accomplished till after the passage of the Jordan. Others think Hormah the name of a town mentioned (Jos 12:14).