ethir is the form of two unrelated Sindarin words (homophones) with different meanings:
"mouth of a river, estuary"
There is some controversy on the pronunciation of the word regarding the digraph th.
One view says that it can be viewed as a compound: ed/et + sîr would trigger a lenition ed/et + hîr. If preposition and stem are understood as different entities, t/h should be pronounced separately ([ˈethir]) (like English "at-home").
Another view would be that PQ etsiri evolved as Old Sindarin *etthire before simplified as ethir. In that case the PQ cluster ts was directly derived as th in Sindarin. The word would be pronounced as [ˈeθir].
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Lost Road and Other Writings, Part Three: "The Etymologies", entry ET
- Jim Allan (ed.) An Introduction to Elvish, "Proto-Eldarin vowels" by Christopher Gilson
- Didier Willis' Sindarin Dictionary version of 2001; containing etymological reconstructions by David Salo
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of Túrin Turambar"