Finer than frog hair
Definition from zikkir, the free dictionary
finer than frog hair
- (simile)(colloquial) Extremely fine.
- 1997 -- Martin Limón: Slicky Boys (page 2) 
- Eun-hi was a business girl. One of the finest in Itaewon. Finer than frog hair, to be exact. A GI's dream...