fmakunbound

(fmakunbound symbol)

Makes symbol name no function again, and returns the symbol. An unknown name is a no-op.

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/#'name/eval through the symbol — because a call site the compiler already bound directly cannot be undone. Built-in macros and special forms are part of the language, not of the image's function namespace, so they are not affected.