She's a small business owner who commutes daily to NYC from New Jersey who came to the US from Japan a few years back.

At the end of class, I asked if I could please be enlightened as to the meaning of 明鏡止水 meikyoushisui。As a personality trait, I not exactly sure what "clear and serene as a polished/clear mirror　明鏡 or still/stagnant water　止水" means in terms of how a person would act in everyday situations. And are there 木、 火、or 金 equivalents?

Thanks 皆さん。

Well, my dictionary supplies "a mind undisturbed [unsullied] by evil thoughts; a serene state of mind" as a definition, if that helps. I'm afraid I can't speak with any authority on your other questions, though.

My dictionary states:

曇りのない鏡と静かな水。なんのわだかまりもなく、澄みきって静かな心の状態をいう。「—の心境」

Looks similar to "tranquil". "A tranquil state of mind"

Yahoo gives this sentence example.

明鏡止水の心境といったところです　
My mind [mental state] is as serene as a polished mirror (and still water).

Ah, nothing like an arcane word to spice the day

明鏡止水 is a beautiful term, but you won't see that one too often. Sadly I've probably heard it more in reference to the alcoholic beverage.
I would say "placid" is a better English translation than just "tranquil" though. 明鏡止水 is very meditative, a calm originating from lack of internal and external stimuli. Peace of mind birthed from silence of thought is how I would put it.

どもありがとう皆さん

Yes thank you thermal3 for this description, its exactly what I was looking for.

曇りのない鏡と静かな水。なんのわだかまりもなく、澄みきって静かな心の状態をいう。「—の心境」

Initiate 馬鹿外人　translation skills!

As with a mirror that's not cloudy... As with a peaceful lake...　Without anxieties or reserve...　A clear and composed　heart and spirit is the state of 明鏡止水さん。

So　can 明鏡止水　be a temporary state of mind? Or is it more a general descriptor of a kind of person?

Does it mean the ability to stay focused? Or can it be construed as apathy to one's environment?　I hope to be able to learn to properly use to use this term　and also appreciate its nuances.

And there's a 明鏡止水　beverage？Wow... sounds deep...　

The Buddha is usually painted sitting with a mirror behind him in meditative absorbtion into the clear light nature of mind. All phenomena are realized as illusionary like the moon's reflection on still water.