A four-character idiom meaning something is never taken outside the household or organization; used for treasured items or secrets kept strictly within and never revealed outside.
その店の伝統のレシピは門外不出とされている。
The shop's traditional recipe is treated as a closely guarded secret (never to leave the premises).
代々受け継がれてきた書物は門外不出の宝物だ。
The books passed down through generations are treasures that are never allowed outside.