If REAL-BINDING is a symbol and has a non-`nil’ `menu-enable’ property, that property is an expression that controls whether the menu item is enabled. Every time the keymap is used to display a menu, Emacs evaluates the expression, and it enables the menu item only if the expression’s value is non-`nil’. When a menu item is disabled, it is displayed in a “fuzzy” fashion, and cannot be selected.

How do I get the property of a symbol? (symbol-plist symbol)