noun, noun which may take the genitive case particle 'no'
never taken out of the premises; kept strictly within the family/group; not for public viewing
Used for treasured items, documents, or secrets that are never allowed to leave a place or be shown to outsiders. Often appears in the pattern 門外不出の〜.
この絵は門外不出の秘宝だ。
This painting is a treasured heirloom that is never taken off the premises.
門外不出のレシピを教えてもらった。
I was taught a recipe that is normally kept strictly within the family.