"orifice, small opening in an animal body," 1680s, Modern Latin, from Greek stoma (genitive stomatos) "mouth; mouthpiece; talk, voice; mouth of a river; any outlet or inlet," from PIE root *stom-en-, denoting various body parts and orifices (source also of Avestan staman- "mouth" (of a dog), Hittite shtamar "mouth," Middle Breton staffn "mouth, jawbone," Cornish stefenic "palate"). Surgical sense is attested from 1937.
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/stomatitis">Etymology of stomatitis by etymonline</a>
Harper, D. (n.d.). Etymology of stomatitis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/stomatitis