Isaiah 21:11 (ESV)

11 The oracle concerning Dumah. One is calling to me from Seir, “Watchman, what time of the night? Watchman, what time of the night?”

Dumah was an oasis in the northern Arabian desert. It was an important oasis for Edom, situated on a main trade route. The word Dumah means silence, but the term is similar in sound to the name Edom. The name Seir confirms that this part of the prophecy is aimed at Edom, as the Edomites lived in this region.