1An adverb expressing that something is to a very great degree — similar to "really" or "truly," often used with a sense of surprise or admiration.
彼の変わりようは何とも驚くべきものだった。
The way he changed was really something to be amazed at.
あの夕焼けは何とも美しい景色だった。
That sunset was a truly beautiful sight.
試験の結果を待つ間、何とも落ち着かない気分だった。
While waiting for the exam results, I felt really restless.
2Used with a negative predicate to mean "not at all" or "can't really say" — commonly seen in the set phrase 何とも言えない ("I can't really say").
うまくいくかどうか、今は何とも言えない。
I can't really say whether it will go well or not right now.
彼にそう言われても、何とも思わなかった。
Even when he said that to me, I didn't feel anything at all.
その味は何とも表現しがたい。
That flavor is hard to describe at all.