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
Arossiach
noun. fords of Aros
arossiach
place name. Ford(s) of Aros
Elements
Word Gloss Aros iach “ford”
Arossiach
Fords of Aros
Aros (river name) + #iach (“ford”)