A function can be specified via the :enabled-function initarg (inherited from menu-object), that determines whether or not the menu, menu item, or menu component is enabled. By default, a menu object is always enabled.
Consider the following example:
(defvar *on* nil)
(contain
(make-instance 'menu
:items
(list
(make-instance
'menu-item
:title "Foo"
:enabled-function
#'(lambda (menu) *on*))
(make-instance
'menu-item
:title "Bar"))))
Figure 8.7 A menu with a disabled menu item
Changing the value of *on* between t and nil in the Listener, using setq, results in the menu item changing between the enabled and disabled states.
By default, items in the menu bar menus and sub-menus are disabled while a dialog is on the screen on top of the active window. You can override this by passing a suitable value for the menu-item initarg :enabled-function-for-dialog.
CAPI User Guide and Reference Manual (Windows version) - 3 Aug 2017