Given its placing at a river's mouths, we can probably interpret nin(d)- as 'water', and in Elvish place-names the closing element -os is usually 'walled town'. The central element -am- is less certain, but seems to be 'up', so the entire name Nindamos translates literally as the rather puzzling 'water-up-town'. Assuming that the elements are translated correctly here, this is perhaps intended to convey something like 'town above the river' or 'town on the sea'.