This is what the Nandor called themselves, a cognate of Quenya Lindar (Teleri) (WJ:385). The sg. is probably *lind, perhaps attested in the name Lindórinan. This form is stated to descend from the older clan-name Lindai (WJ:385), or at the oldest stage Lindâi (WJ:378). Lindâ was originally the name of a member of the Third Clan of the Elves, among the Eldar also called the Teleri; the Nandor came from this branch of the Eldarin peoples. In WJ:382, Lindâ is stated to be derived from a stem LIN, the primary reference of which is to "melodious or pleasing sound"; Lindâ, derived by medial fortification and adjectival -â, would seem to be in its origin an adjective, but later applied to the third clan of the Elves and eventually used as a noun. The reference was to their love of song (notice that Tolkien translated the name Lindórinan as "Vale of the Land of the Singers"; UT:253).
The Nandorin word Lindi alone in our small Green-elven corpus shows a direct descendant of the Primitive Quendian ending -î, while the sole other attested Nandorin plural is formed by umlaut: urc "Orc" pl. yrc. Perhaps the ending -i persisted in the case of words that had the stem-vowel i, since this vowel could not be changed by umlaut (being already identical to the vowel causing the umlaut so that no assimilation was possible); therefore, singular and plural would become identical if the plural ending -i had been dropped as in yrc. (It may not be necessary to invoke the simple "real-world" explanation that Tolkien's ideas about Nandorin had changed during the thirty years that separate the source that has yrc from the source that provides the word Lindi.)
@@@ as suggested by Lokyt, possibly a plural form of unattested log, since it is glossed in the plural in the source material: “pools”