1.
indescribably; somehow; ever so
With an affirmative predicate, marks a feeling the speaker cannot quite put into words. Very common in なんとも言えない (indescribable) and before an adjective of impression.
2.
not at all; nothing much; no problem
With a negative predicate, denies the whole of something — that anything is wrong, that anything can be said, or that anything is felt.
From 何 + と + も. Which sense applies is decided entirely by whether the predicate is affirmative or negative, so read to the end of the clause before interpreting it.
Common patterns
- なんとも言えない: indescribable; impossible to say either way
- なんとも思わない: to think nothing of it, to be unbothered
- なんともない: to be fine, to be nothing serious
- なんとも申し上げられない: I am not in a position to comment (formal)
Usage
Sense 1 before an adjective is a warm, slightly literary intensifier: なんとも美しい (ever so beautiful). Sense 2's なんともない is the standard reply to "are you hurt?" or "is anything wrong?".