The best known Quenya word for “city”, but strictly speaking really a fortification or a stronghold (Ety/OS; MR/350; NM/228; PE22/124; WJ/414). The two were more or less synonymous, since in Middle-earth most cities were fortified. In theory osto might also be used of any large fortification, not just a fortified city, as was the case with its Sindarin cognate ost, but in most of the Quenya examples it was used in city-names.
Conceptual Development: Possible precursors include the words ᴱQ. os (ost-) {“dwelling, hamlet” >>} “cottage, house”, ᴱQ. osta {“walled tower” >>} “homestead”, and ᴱQ. ostar {“walled tower” >>} “township” in the Qenya Lexicon of the 1910s under the early root ᴱ√OSO [’OSO] (QL/71). The contemporaneous Poetic and Mythological Words of Eldarissa (PME) had only the older glosses, but the forms began with h: ᴱQ. hos(t) “dwelling, hamlet”, ᴱQ. hosta/hostar “walled tower” (PME/71). Elsewhere in PME Tolkien said that osta was equivalent to ᴱQ. irin “town” (PME/43).
The Etymologies of the 1930s had ᴹQ. osto “city, town with wall round” under the root ᴹ√OS “round, about” (Ety/OS). Notes on The Feanorian Alphabet from the 1940s had osto “fort” (PE22/50 note #183), but in the Quenya Verbal System of the late 1940s Tolkien glossed this word as “city” in the phrase: tasse i·osto “there (is) the city” (PE22/124).
In Tolkien’s later writing this word was glossed “fortification” (NM/228), “a strong or fortified building or place” (MR/350), and “fortress or stronghold” (WJ/414). With some exceptions like the name Mandos, it appeared mainly as an element in the names of cities of Men, Elves or Dwarves, such as Armenelos, Formenos, and Túrosto. Note the reduction of the suffix to -os in longer compounds, but not in shorter compounds like Túrosto.
irin noun "town" (LT2:343; hardly a valid word in Tolkien's later Quenya)