before vowels mast-, word-forming element meaning "female breast, mammary gland," from Greek mastos "woman's breast," from madan "to be wet, to flow," from PIE *mad- "wet, moist, dripping" (source also of Latin madere "be moist;" Albanian mend "suckle;" see mast (n.2)).
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/mastitis">Etymology of mastitis by etymonline</a>
Harper, D. (n.d.). Etymology of mastitis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/mastitis