I think "predicate" is the original meaning of that postfix. Emacs Lisp, at least, can use question marks in names without batting an eyelash. I see no reason

I think "predicate" is the original meaning of that postfix. Emacs Lisp, at least, can use question marks in names without batting an eyelash. I see no reason that would not have been true for Lisps generally since their birth.