Googling tells me this: https://stackoverflow.com/questions/4982732/what-word-is-abbreviated-by-n-in-names-of-destructive-lisp-functions
which tells me this: https://gigamonkeys.com/book/they-called-it-lisp-for-a-reason-list-processing.html#destructive-operations --- via emacs-tangents mailing list (https://lists.gnu.org/mailman/listinfo/emacs-tangents)