noun
tiny amount; the slightest bit
Emphatic expression for an extremely small quantity, often used in negative or conditional contexts to mean 'not even a tiny bit'.
彼の話には一糸一毫の真実もない。
There is not a shred of truth in his story.
一糸一毫たりとも無駄にできない。
We cannot waste even the slightest bit.
僅か means 'a small amount' in a neutral sense; 一糸一毫 emphasizes the extreme minuteness, often with a rhetorical tone.
A yojijukugo composed of 一 (one), 糸 (thread), 一 (one), and 毫 (fine hair), literally 'one thread, one fine hair', metaphorically indicating an extremely tiny amount.