[lispm]

March 1960, the Lisp I Programmer's Manual:

  ff[x] = [atom[x] -> x; T -> ff[car[x]]]

or

  DEFINE ((
    (FF (LAMBDA (X)
          (COND ((ATOM X) X)
                (T (FF (CAR X))))))
    )) ()

which one or two decades later would be in Lisp:

  (defun ff (x)
    (cond ((atom x) x)
          (t (ff (car x)))))