Tafsir of Ad-Dukhan 44:24

That is, still/calm, as Ibn Abbas said. It is said, "The sea raha (became calm), yarhu, rahwan," meaning it became still. It is also said, "The horses came rahwan," meaning calmly/steadily. The poet said:

And the horses proceed steadily (rahwan) In their reins, like birds escaping a sudden hailstorm.

It is also said, "Do that rahwan," meaning calmly and at a leisurely pace. More than one authority cited the lines of al-Qutamī describing camels:

They walk steadily (rahwan), such that the rumps do not fail, Nor do the chests lean upon the rumps.

The apparent meaning is that it is originally an infinitive (masdar) interpreted as an active participle. It is permissible for it to mean "the actually still [thing]."

From Mujahid: rahwan means split open and widened. Abu Ubaidah said: "A man raha, yarhu, rahwan," meaning he opened his legs wide. It is reported from some Arabs that they saw a camel that was faljan—meaning possessing two humps—and said: "Glory be to Allah the Exalted, a rahw (gap/space) between two humps." They meant a wide opening. The apparent reading here is also that it is an infinitive interpreted as such, or that there is an implied genitive construct, meaning "possessor of a gap."

Qatadah said: Moses, peace be upon him, intended, after he and those with him had crossed the sea, to strike it with his staff so that it would close up, just as he had struck it at first so that it would split, so that Pharaoh and his troops would not follow him. He was ordered to leave it rahwan—meaning open and split, or still in its state, remaining as it was with the water standing and the path remaining dry. He was not to strike it with his staff or change anything about it, so that the Copts might enter it. When they were all within it, Allah the Exalted would close it upon them. This is the meaning of His saying: "Indeed, they are an army to be drowned."

This is the justification for the command to leave it rahwan. It has been said: rahwan means easy; it has been said it means dry; it has been said it means a solid path; and other things have been said. All of these are explanations of the resulting meaning.

Al-Raghib claimed that the correct meaning is the wideness of the path. He then said: "From this is al-raha’, the level wilderness. It is said for every level depression in which water gathers, rahw. From this it is said, 'There is no right of pre-emption (shuf’ah) in a rahw (a shared, open space).'" The truth is that what he mentioned is among its various usages, but as for it being "the correct [definition]," that is not the case.

It was recited as annahum (that they are) with an alif (fathah), meaning: "because they are."