(or 中々（なかなか） in kanji) is a 副詞（ふくし） (adverb) in Japanese that is frequently translated as ‘very’, or ‘considerably’. It is regularly paired directly with other adjectives, or before nouns (with の coming between なかなか
and the noun).
This ramen shop is pretty good.
Mimura-san is very cute.
The pasta that the chef makes is very good.
My ex-girlfriend was considerably beautiful, but her personality was bad, so I broke up with her.
can also sometimes have the nuance of being something that is 軽視（けいし）しがたい
. This basically translates as ‘difficult to dismiss’, but is a bit closer to English expressions like ‘nothing to scoff at’, or ‘not to be taken lightly’.
You have done something that is difficult to dismiss, haven't you? How am I supposed to explain this to our client? (What you did cannot be taken lightly. How am I supposed to explain what has happened to our client?)
highlights that something is ‘considerably/extremely (A)’, or ‘far more (A) than expected’. If we imagine a dartboard, where the whole board shows the intensity level of a specific word, and なかなか
represents the bullseye, this is basically the nuance. ‘(A) is the middle (bullseye) example of (B)’.
なかなか, coming from the kanji 中（なか） meaning ‘within’, can be thought to mean the 'middle' or 'most' (A).