well-known to those in the know; known to connoisseurs
A set phrase used to describe something that is highly regarded or famous within a specific, often niche, group, but not widely known to the general public. Often used to praise hidden gems, such as restaurants, products, or experts.
This ramen shop is a famous establishment known to those in the know.
He is a skilled craftsman known to connoisseurs.
A set phrase composed of 知る (to know), 人 (person), and the emphatic particle ぞ, literally 'those who know, know'. The exact origin is uncertain, but it has been used since at least the Edo period to refer to things appreciated by a discerning few.