"(Seriously. "cdr"? "endp"? Was there a Great Keystroke Shortage in the '60s?)"

To be fair, CL has "first" and "rest" as well as "car" and "cdr". And as already pointed out, "endp" makes perfect sense if you want to establish a convention for predicates but don't want to allow symbols to contain special characters like ? marks.

(I know that CL allows symbols to contain ?s if you surround with with || but I don't think anyone seriously uses something like (defun |end?| (x) ... )