[na-adj] + でも何でもない
The expressions でも何でもない and くも何ともない are used to strongly deny something, emphasizing that something is "not … in the least," "nothing of the sort," or "not at all." They convey a forceful rejection of a characterization, feeling, or state.
This pattern attaches to nouns or na-adjectives to emphatically deny that something is what it is claimed or assumed to be. It literally means "not even anything (of that sort)."
The nuance is that the speaker wants to completely dismiss the idea that the noun/adjective in question applies at all. It is stronger than simply saying ではない.
This pattern attaches to the stem of i-adjectives (removing the final い) or to verbs in their く or conjunctive form, and strongly denies a feeling, sensation, or state. It means "not … in the slightest" or "don't feel … at all."
Both expressions belong to emphatic denial and are common in spoken Japanese and informal writing. They carry a dismissive, sometimes defiant tone – the speaker is pushing back against an assumption or expectation. Because of this strong tone, they are most natural when the speaker is correcting someone else's impression or responding to a situation where a certain characterization has been implied.
でもなんでもない何でもない is used with **nouns and na-adjectives**, while くもなんともない何ともない is used with **i-adjectives** (in their く form). Mixing up the two patterns is a common mistake.