爪の垢を煎じて飲む literally means “to make a brew from the dirt of one’s fingernails and drink it”. It’s a very gross expression, isn’t it? However the closest English expression for 爪の垢を煎じて飲む would be “to take a leaf out of someone’s book” - we use this weird expression when we admire someone and want to be like that person.
In old times, when science was not developed and medicines were made from weird things like lizards and cat whiskers, some people believed we could gain the abilities and talents of a person if we drank medicine made from the dirt under the person’s fingernail!
爪の垢を煎じて飲む works as a verb. Since we use this expression when we have a desire to become like the person, often the expression combined with the grammar ～たい (want to do), e.g. 飲みたい (want to drink). If we want John to learn from someone they admire, Susan, we can say JohnにSusanの爪の垢を煎じて飲ませたい。 It’s also ok to omit the word 煎じて (to make a brew / decoct) and shorten the phrase to just “爪の垢を飲む .
Joe is fluent in Japanese. I want to be just like him.
Takeshi is always polite, but my kid is…. well, I’d like my kid to be jut like Takeshi.