§ Caerost on Kiril (WR:437)
§ Tarnost (ibid.)
These are names of two cities in Gondor.
Caerost preceded Calembel (see Calledin, Calembrith above (3.32)). It obviously contains N. ost ’city, town with wall around’ (OS-). Caer is attested in Noldorin as the numeral ’ten’ (KAYAN-, KAYAR-), thus *’ten-town’? Else it might be a derivative of KAY- ’lie down’, whence N. caew ’lair, resting-place’, thus *’lying/resting town’.
Tarnost was a pencilled addition to the map, located on the river Ringlo. The initial element #tarn- is the same as in earlier Tarn Felin (TI:424), see above (2.68). Taking Early Qenya tarna ’crossing, passage, ford’ into account, as well as the town’s position on a river, it might indeed be the *’crossing-town’ or *’ford-town’. Compare also THAR- ’across, beyond’, thar- ’athwart, across’ (Silm.index), whence another city name is derived: Tharbad < thara-pata ’cross-way’. See also Bronwe athan Harthad ’Endurance beyond Hope’ (SD:62), where athan may be a misreading for *athar ’beyond’ < THAR-. In Quenya the preposition tar ’beyond’ occurs once (LR:72), rather then *sar from THAR-.
It is hardly more than a guess, but maybe there was a parallel stem *TAR-, likewise meaning something like ’across, beyond’, yielding the Qenya preposition tar, but the Noldorin/Sindarin noun #tarn *’crossing, ford’. Note also the frequent occurrence of English ford, German Furt, Dutch voorde in place names.
But we also find a explicitly attested root √TĂR ’stand, intransitive’ (PE17:186), so that tarn *’standing’ could be an adjectival derivation from it. Hence maybe Tarnost *’standing town’ in symmetry to Caerost *’lying/resting town’.