not in the least; nothing like that
Expresses that something is absolutely not the case, emphasizing a strong negation.
This pattern is used to strongly deny that something is a certain thing or quality, often in response to a suggestion or assumption. It carries a sense of 'not in the least' or 'nothing like that'. It is emphatic and somewhat colloquial, though not rude, and is more common in spoken Japanese or in informal writing.
When used with nouns, it attaches directly to the noun followed by でも何でもない. For example, 天才でも何でもない means 'not a genius or anything like that'. For na-adjectives, use the stem in the same way. For i-adjectives, the stem is used with くも何ともない, as in 難しくも何ともない, meaning 'not difficult at all'.
The nuance is often that the speaker is downplaying or rejecting an exaggerated claim. It can also be used to reassure someone that something is not as bad as they think.