"a fold of the peritoneum," early 15c., mesenterie, from medical Latin mesenterium "middle of the intestine," from medical Greek mesenterion, literally "the middle intestine," from mesos "middle" (from PIE root *medhyo- "middle") + enteron "intestine" (from PIE *enter-, comparative of root *en "in"). Related: Mesenteric.
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.