An island in the river Anduin (LotR/812), translated “Ship of Long Foam” (PM/371), sometimes appearing as just Andros “Long Foam (LotR/1115). This name is a combination of cair “ship”, and “long” and ross “foam, spray” (SA/an(d), ros).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this island was first named N. Tol Varad “Defended Isle”, soon revised to Men Falros “✱Place of Splashing Foam” (WR/326). Later in the drafts it was changed to N. Cairros “✱Ship Foam” then Andros before Tolkien settled on its full form N. Cair Andros (WR/340 note #15).
The Sindarin word for “ship”, most notably appearing as an element in the name Cair Andros “Ship of Long Foam” (LotR/812; PM/371). It is derived from primitive ✶kiryā, with the ancient i becoming e via a-affection [kery(a)], then the y intruding into the main syllable to form the diphthong ei [keir], and ultimately ei becoming ai as usual in final syllables in Sindarin of the 1950s and 60s [kair]. This word has a somewhat unusual plural, since ī replaced final ā in its ancient plural [kiryā-ī > kirī], so that a-affection did not occur resulting in a modern plural form cîr “ships” (PE17/147). Its class plural is likewise the somewhat unusual ciriath “[all the] ships” for similar reasons.
Conceptual Development: The Etymologies from around 1937 had N. ceir “ship” under the root ᴹ√KIR “cleave” (Ety/KIR), since in Noldorin of the 1930s ei did not (usually) become ai in final syllables. In Primitive Quendian Structure: Final Consonants from 1936, Tolkien gave cīr “ship”, first marked “N.”, then “Ilk.”, then “N. & Ilk.” (PE21/57 and note #28). It had the class plurals círiath or ciriath but it is not clear which of these was the intended final form (PE21/57 note #28). I think ciriath is more phonologically plausible; compare class plural S. Firiath “Mortals” vs. ordinary plural Fîr (WJ/387).
Early Noldorin Word-lists of the 1920s had the word ᴱN. cair followed by ᴱN. braithgair, but neither word was translated (PE13/139-140).