The Buddha dies at Kusinārā, a humble hamlet that at one time had been the grand city of Kusāvatī, capital of King Mahāsudassana (Great Glory). Kusāvatī was built of gold and sparkling gems, shining like … well to be honest, shining rather like a modern city of metal and glass.
But the word kusa means “grass”, specifically a kind of rather spiky and common grass that was employed in the Brahmanical rituals. It seems rather a leap from common grass to glittering gems.
One of the greatest of the Jātaka stories, is the Kusajātaka (Ja 531), which tells of Kusa, the ugly yet wise son of the legendary King Okkāka, and how he won not just his kingdom but also the hand of his fair lady Pabhāvatī. It’s a stunning story, which constantly associates the grand king with positions of humility and disgrace. His city is, of course, Kusāvatī. The commentary tells us this was the seat of the Mallas, thus confirming that for them it was identical with the Kusinārā/Kusāvatī of DN 16.
The Rāmāyana tells us of another (?) Kuśa, this time the son of Rāma. He settles in another Kusāvatī, which is located in the Vindhya ranges, known in Pali as Dakkhinagiri, far to the south. Nonetheless, this was said to be part of Kosala, which as far as we know was, rather, to the west of the Kusāvatī in the Mallian realm. The Rāmāyana is of course much later, and this detail is in the last chapter, which is usually said to be a later addition by another hand, so perhaps we should not place too much stock in this.
Are the two Kusas and their cities, in fact, the same? In both cases, their name stems from the fact that spiritual assistance granted their royal conception, facilitated by kusa grass. Clearly there is an association of royal lineages being certified by rituals involving kusa in cases where paternity is in question—which TBH is pretty much all the time. They may draw from the same mythic roots, or simply be named for similar customs.
Now, according to Wikipedia, the 15th century supplement to the Rāmāyaṇa called Ānanda Rāmāyaṇa supplies more details about the city. I can’t locate the text to confirm this. But it apparently says that the city was deserted and fallen to ruins, and Kusha returned it to its former glory.
This does rather recall the lost city of Kusāvatī, reduced to a forgotten hamlet called Kusinārā. And it suggests a possible source for the name: a ruined city covered in grass.
However, the closeness of the names kosala and kusāvatī perhaps suggests another connection. The Dictionary of Pali Proper Names says the following about the origin of the name Kosala:
The Commentaries (E.g., SNA.ii.400f; DA.i.239f) give a curious explanation of the name Kosalā. It is said that when nothing could make Mahāpanāda smile, his father offered a big reward for anyone who could succeed in doing this. People, accordingly, left their work and flocked to the court, but it, was not until Sakka sent down a celestial actor that Mahāpanāda showed any signs of being amused. When this happened the men returned to their various duties, and on their way home, when meeting their friends, they asked of each other, “Kacci bho kusalam, kacci bho kusalam.” The district where this occurred came to be called Kosalā on account of the repetition of the word kusala.
If this explanation seems unconvincing, perhaps we could connect kosala with kusa, in which case it would have basically the same meaning as kusāvatī, i.e. “full of grass”. Note that it is sometimes spelled kośala in Sanskrit, which would agree with a derivation from kuśa.
Why grass though? And why ruined cities? The memory of ancient ruined cities of a legendary past was probably informed by the decaying cities of the Harappan civilization, which had been abandoned a thousand years earlier. The major Harappan centers were to the north-west, but it seems they created a strong cultural imprint.
Perhaps Kosala simply means “land of grass” and was a region rich in grassy fields. But I would connect the prominence of grass with the Vedic ritual. Grass is laid upon the altar to signify the start of the ritual. It is a humble bed for the glorious gods.
Kosala became a prominent center for Brahmanical culture, so perhaps the ultimate source of its name is “the place where grass was laid” as the basis for Vedic culture in the region. Specifically, where kusa grass played a prominent role in establishing the authority of kings by brahmins according to Vedic custom.