1540s, "subjoined addition to a document or book," from Latin appendix "an addition, continuation, something attached," from appendere "cause to hang (from something)," from ad "to" (see ad-) + pendere "to hang" (from PIE root *(s)pen- "to draw, stretch, spin"). Used for "small outgrowth of an internal organ" from 1610s, especially in reference to the vermiform appendix. This sense in English is perhaps from or influenced by French appendix, where the term was in use in anatomy from 1540s.
<a href="https://www.etymonline.com/word/appendices">Etymology of appendices by etymonline</a>
Harper, D. (n.d.). Etymology of appendices. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/appendices