Latin 'to hang upon' — attaching by dangling something onto something else, evolved into adding to a list.
To add something to the end of a document, piece of writing, or list; to attach or hang onto something.
From Latin 'appendere' (to hang upon, hang to, weigh out), composed of 'ad-' (to, upon — assimilated to 'ap-' before 'p') and 'pendere' (to hang). The literal meaning was 'to hang something onto something else' — to attach by suspending. The sense shifted from physical attachment to textual attachment: appending a note to a document is metaphorically hanging it onto the end. The related 'appendix' (from Latin 'appendix,' something hung on) entered English earlier. Key roots: ad-/ap- (Latin: "to, upon, toward"), pendere (Latin: "to hang, to weigh").
Your anatomical appendix is etymologically 'something hung onto' the large intestine — a small pouch dangling from the cecum. When a book has an appendix, it too is something hung onto the main body. The vermiform appendix was so named because it looked like a small appendage hanging from the gut, and the bookish sense preceded the anatomical one by centuries.