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.
word-forming element in medicine denoting "diseases characterized by inflammation" (of the specified part), Modern Latin, from Greek -itis, feminine of adjectival suffix -ites "pertaining to." Feminine because it was used with an implied nosos "disease," a feminine noun; especially in arthritis (nosos) "(disease) of the joints." Arthritis (16c.) was one of the earliest appearances of the suffix in English and from it the suffix was abstracted in other uses.
<a href="https://www.etymonline.com/word/appendicitis">Etymology of appendicitis by etymonline</a>
Harper, D. (n.d.). Etymology of appendicitis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/appendicitis