noun
secret menu; off-menu item
Refers to dishes or services that are not listed on the regular menu but can be ordered by those in the know. Often used in restaurants, bars, and other service establishments.
この店には常連だけが知っている裏メニューがある。
This restaurant has a secret menu that only regulars know about.
裏メニューのラーメンはすごく辛いらしい。
I heard the off-menu ramen is incredibly spicy.