وَثَمُودَ ٱلَّذِينَ جَابُوا۟ ٱلصَّخْرَ بِٱلْوَادِ
Root: ج-و-ب. Its meaning is to cut, to pierce, to hollow out, to pass through the inside of something. Jāba'l-arḍa — he traversed the land, he crossed it.
The concrete flavour of the word is this: to enter into a mass and open it. Not merely to cut; to bring a road or a hollow out of its inside.
To establish this link is instructive. A jawāb is what cuts through and brings the counterpart of a question; ijāba and istijāba are from the same root as well — to respond to a call. The root's common idea is to come opposite a thing and meet it: to come opposite the rock and split it, to come opposite the question and meet it.
I record the observation that follows from this as my own reading and in a measured way: Thamūd was a people that could answer the rock; it could not answer what was said to it. Two works from the same root — one done, one not done. I do not present this as a claim of rhetoric; I record it as the echo the word carries.
ٱلْوَاد (al-wād) — a valley. The original of the word is الوادي (al-wādī), with a yāʾ at the end. In the verse the yāʾ has dropped: al-wād.
This is the same phenomenon as yasri in the fourth verse: the dropping of the final yāʾ for agreement of verse-ending. The sūra performs the same operation twice, and in both the reason is the same: the settling of the verse-ends onto the sound -ād / -r.
I should like to add an observation, as my own reading: the nine verses in this part of the sūra (6-14) close almost entirely with the same sound — ʿĀd, ʿimād, bilād, wād, awtād, bilād, fasād, ʿadhāb, mirṣād. Eight of the nine end with the sound -ād. So tight a unity of sound makes the section a single block: three peoples, their transgressions and their recompenses, stand in the ear as a single chain.
The story of Thamūd was treated fully in Sūrat al-Shams: what the Qurʾān says, what the reports add, and the historical frame were given separately. I do not repeat it.
Among the things recorded there this verse already occurred: "They made houses by hewing the mountains (al-Aʿrāf 7/74; al-Ḥijr 15/82; al-Shuʿarāʾ 26/149). In Sūrat al-Fajr they are mentioned as 'those who carved out the rocks in the valley' (al-Fajr 89/9)." I close that link from here.
The only new thing belonging here is the choice of the word. Elsewhere the Qurʾān says yanḥitūna mina'l-jibāli buyūtan — "they hew houses out of the mountains." Here the verb is jābū: they carved, they cut through. There is no word for house, only the rock and the verb. The sūra is concerned not with what the structure was but with the rock's having been split.
This choice is meaningful, because in this part of the sūra all three peoples are mentioned by the work they did, not by the life they built.
I need to repeat here the note entered in the section on al-Shams, because this verse of al-Fajr is the verse where that note is most needed. The expression "those who carved out the rocks in the valley" is the sentence most open to being directly matched with the rock faces photographed today.
The Qurʾān's description "they made houses by hewing the mountains" is associated with the rock-cut tomb façades in the region of al-Ḥijr (today Madāʾin Ṣāliḥ) in north-western Arabia. Care is needed here: the monumental façades standing today belong to the Nabataean period (roughly the 1st century BC – 1st century AD). Thamūd, however, is a community dated far earlier than that. That is, the tradition of rock-carving in the region shows a continuity, but the structures tourists see today are not the structures of Thamūd.
This distinction must be preserved. To say "here are the houses of Thamūd" is easy and impressive; but it is wrong, and when the error is discovered it damages the text as well.
The second thing recorded in the section on al-Shams holds here too: Thamūd was not a legendary name for those addressed by the Qurʾān. The caravan road from Mecca to Syria passed through the region. The sūra is not telling a tale; it is speaking of a ruin the one addressed saw on his way.