[Mithalwen]

There might be a much more prosaic explanation, I am not a great Elvish scholar, I have noticed that Tolkien often gives descriptive names for his female characters... With Arwen, after a few try outs he just calls Elrond's daughter "Noble maiden", which is what she is. Erendis means " Lonely bride", Aredhel "noble elf". I know we have a tendancy to analyse everything to the nth degree, but maybe when he needed a name for this minor character, eldest of the wise women - he simply called her "ancient" (which of course does not have competely negative connotations (cf the elvish name for Bombadil, Iarwain - rendered as "eldest").

After all, at the time LOTR was published, I don't suppose that he ever expected that many people would find out waht these names meant.