Tafsir of Al-Kahf 18:94

{Yājūj and Mājūj} They are two non-Arabic names, evidenced by their being diptotes (ghayr munṣarif). It has been recited with a hamza (i.e., Ya’jūj and Ma’jūj). Ru’bah recited them as Ājūj and Mājūj. They are descendants of Japheth. It is said that Yājūj are the Turks, and Mājūj are the Jīl and Daylam.

{Mufsidūna fī al-arḍ} (Corruptors in the land) It is said: They used to eat people. It is also said: They would emerge during the spring, leaving nothing green without eating it, and nothing dry without carrying it away. They would inflict killing and severe harm upon those they encountered.

[Description of them] It is narrated from the Prophet (ﷺ) regarding their description: "None of them dies until he sees a thousand males from his own loins, all of whom have borne arms." It is said: They are of two types—some excessively tall, and some excessively short.

[Linguistic Notes] It has been recited as kharjan and kharājan, meaning: "We shall set aside for you a portion of our wealth." Similar to this are the words nawl and nawāl. It has also been recited as saddan and suddan, with both the fatḥa and the ḍamma.

{He said: "That in which my Lord has established me is better, so assist me with strength; I will make between you and them a barrier. Bring me sheets of iron." Until, when he had leveled [them] between the two mountain walls, he said: "Blow," until when he had made it [like] fire, he said: "Bring me, that I may pour over it molten copper." So they were unable to scale it, nor were they able to pierce it.}