Is it a verbal body of water? - (Gosh, I have no idea what you mean by this but I'll make one final try):
Not Aelin-uial
Is it a name of a great greyness to the north? -
Not Angband
Is it part of a command used by a great Elven-lord? -
Not 'Ai' as in 'Ai na vedui, Dunadan...' Eh, that's probably not it.