dirt under one's fingernails
Literal meaning: the grime that accumulates under fingernails. Used in the set phrase 爪の垢を煎じて飲む (to learn from someone's example).
There's dirt under my fingernails.
I wish I could learn from his example (literally: boil and drink the dirt from under his nails).
Idiomatic use: a tiny amount of a desirable quality, often in the phrase 爪の垢でも煎じて飲め (you should learn even a little from someone better).
If you could learn even a shred from him, you'd be a bit better.