でも何でもない / くも何ともない【でもなんでもない / くもなんともない】

でも何でもない and くも何ともない deny something flatly: "not … at all", "nothing of the kind". でも何でもない follows a noun or a な-adjective; くも何ともない is the い-adjective version, with the い changed to く.

Which form

The word in front decides, just as it does between ではない and くない. The middle word changes too: 何でも after で, 何とも after く. いい becomes よく, as it does before ない. Both are often written in kana: でもなんでもない, くもなんともない.

• × 痛いでも何でもない → 痛くも何ともない
• × 好きくも何ともない → 好きでも何でもない

Pushing back

Both answer an assumption, spoken or implied: someone thinks it hurt, or that he is your boyfriend, and you deny it outright. The tone is dismissive, sometimes defiant, and both belong to speech and informal writing. The next sentence often says what the thing really is.

Word for word it is "not X, and not anything": the denial throws out the word and everything like it. ではない and ぜんぜん全然きごう〜くない state a plain fact, and they are what a neutral report uses.

Usage notes

These go beyond the plain present.

Past and polite forms

The final ない changes like any ない: なかった for the past, ありません or ないです for polite speech.

Linking to a second clause

でも何でもなくて, or でも何でもなく in writing, links to a clause that says what the thing is instead.