فَإِن يَصْبِرُوا۟ فَٱلنَّارُ مَثْوًى لَّهُمْ وَإِن يَسْتَعْتِبُوا۟ فَمَا هُم مِّنَ ٱلْمُعْتَبِينَ
"Even if they endure, the Fire is their dwelling; and even if they seek to win favour, this chance is not granted them."
يَسْتَعْتِبُوا۟ — root ع-ت-ب. The core the lexicographers give: to apologise in order to remove anger, to try to bring the other side round. Iʿtāb — to accept the apology and be satisfied; istiʿtāb — to seek that acceptance.
The concrete branch of the word is recorded as well: ʿataba, the threshold of a door and the step of a stair. The lexicographers explain the link with the picture of "the removal of an obstacle placed in between". I transmit this relation in the form in which the lexicographers establish it.
مَثْوًى — root ث-و-ي: to remain in a place, to reside. The word means "dwelling, the place where one stays" — not a temporary stopping-place.
The construction of the verse closes off both possibilities: enduring (yaṣbirū) and apologising (yastaʿtibū). I record this as an observation: this block of the sūra states, with a sentence that runs in two directions, that responsibility cannot be deferred — neither by bearing it out nor by turning back.