How to undefun a function in Emacs Lisp?

I was looking for how to make defined function void in Elisp. Now I have the answer!

(defun foo (x) x)
=> foo
(foo 1)
=> 1
(fmakunbound 'foo)
=> foo
(foo 1)
error--> Symbol's function definition is void: foo

You can use makubound function for variables, instead.