adverb
(not) one; (not) a single
Used with a negative predicate to mean 'not even one' or 'not a single thing'. Emphasizes complete absence.
何一つ問題はありません。
There is not a single problem.
彼は何一つ知らなかった。
He didn't know a single thing.
Standard kanji-kana mixed spelling.
Variant with hiragana ひとつ; equally common in informal writing.
何も is a more general 'nothing' or '(not) anything', while 何一つ emphasizes 'not even one' and is stronger.
Compound of 何 (nani, 'what') + 一つ (hitotsu, 'one thing'). The reading なにひとつ is a straightforward combination.