A ford across the river Aros, translated “Ford(s) of Aros” (S/121, WJ/338). It is a compound of the river name Aros and the (singular) noun iach “ford”.
Sindarin
aros
place name. Aros
arossiach
place name. Ford(s) of Aros
Arossiach
noun. fords of Aros
Aros (river name) + #iach (“ford”)
Aros
Aros (name)
The word Aros, derived from the Elvish root YAR- ("blood"), is said to be a "name of river with reddish water". The Noldorin form of the word was iaros.
Arossiach
Fords of Aros
A river in southern Doriath (S/121). The meaning of this name is unclear.
Conceptual Development: This name dates back to the earliest Lost Tales, where the name G. Aros appeared (LT2/236). In The Etymologies from the 1930s, Tolkien designated this name as Ilkorin Aros, related to the adjective Ilk. arn “red” due to the colour of its waters (Ety/YAR). Since Tolkien abandoned the Ilkorin language, it is unclear what the meaning of this name would have been in his later writings.