One such case that I recently came across is ḍarīʕ ضريع, occurring in the Qur'ān only once, in verse 6 of Surat Al-Ghāshiyah:
They [the inhabitants of Hell] have no food except ḍarīʕ, which neither fattens nor takes away hunger.The commentators' consensus is that this is a Ḥijāzī word, unfamiliar to Arabs from other regions, referring in this passage to dried shibriq – a thorny shrub with the Latin name of Zilla spinosa. The obscurity of this term outside the Qur'ān may be gauged by the fact that many early Arabic dictionaries omit it entirely; almost all occurrences in Alwaraq.net's rather large collection of classical Arabic literature are in quotes or explanations of this Qur'ānic verse.
So far, I am not aware of any Arabic dialect in which a reflex of the word has survived in the Qur'ānic sense. However, the names of land plants are very often extended to sea plants – for example, Ulva lactuca is “sea lettuce” in English, “laitue de mer” in French, and šḷađ̣a taʕ əlbħəṛ شلاظة تاع البحر in Dellys Arabic – and ḍarīʕ appears to be a case in point. Ibn al-Bayṭār (a 13th century botanist born in Málaga) glosses ḍarīʕ simply as a plant cast up by the salt sea from its bottom, found along the sea coast, not even bothering to mention the Qur'ānic usage of the word. The 13th century lexicographer Ibn Manđ̣ūr, born in Tunis, likewise gives as the primary meaning of ḍarīʕ “a green, stinking, light plant cast up by the sea”. [Addition: A more picturesque attestation occurs in al-Nuwayrī (Egypt, 13th-14th c.), who describes a Fatimid general's conquest of Morocco: "He continued until he reached the ocean, and ordered that some fish be caught, and put them in a jar of water and brought them to Mu`izz in the post, and put inside his letter ḍarīʕ of the sea."]
Unlike the terrestrial meaning, this sense has survived in colloquial usage right up to the present day: in Dellys (central Algeria), the keystone seagrass of the Mediterranean, Posidonia oceanica, is called ṭṛiʕ طريع. Just as Ibn al-Bayṭār describes, this seagrass is cast up by the sea in vast quantities along the coast. The change of ḍ > ṭ is not regular in Dellys, but is sporadically attested here (eg ṭəmm “gather together” < ḍamma), and is a good deal commoner in the very similar dialect of the Algiers Casbah; so the derivation is unproblematic.
What is the connection between Zilla spinosa and Posidonia oceanica? It would be hard to think of two plants which resemble one another less. Posidonia oceanica has no thorns, never forms a shrub, and isn't even the same shade of green. Rather than form, we must look to function. Zilla spinosa is (if we may trust the lexicographers and commentators) very bad fodder; Ibn Manđ̣ūr comments that camels fed on it gain neither fat nor meat. Posidonia oceanica has many functions reported for it in Mediterranean cultures, but that of fodder is conspicuous for its absence, suggesting that it too does not make good fodder. This suggests an etymology: the meanings of the root ḍrʕ include “be humble, weak”, and cattle fed on ḍarīʕ presumably become weak.
The obvious follow-up question is whether either sense of ḍarīʕ has survived elsewhere in Arabic, so I'll close by asking my readers: is the word ḍarīʕ used in an Arabic dialect that you know? If so, what does it mean?