fmakunbound

(fmakunbound function-name)

Makes function-name name no function again, and returns it. An unknown name is a no-op. A (setf name) list, quoted or built at run time, retires the (setf name) function.

On the interpreter the global function binding (and any defmacro macro of the same name) is removed outright, so a later call signals The function X is undefined. On the compiled backends the name is retired only for late-bound references — fboundp, funcall/apply/#'name/eval through the symbol — because a call site the compiler already bound directly cannot be undone. A function a defun below the top level defines is retired for direct calls too. Built-in macros and special forms are part of the language, not of the image's function namespace, so they are not affected.