"acute inflammation of the bowels," 1808, medical Latin, coined c. 1750 by French pathologist François-Boissier de la Croix de Sauvages (1706-1767), from enteron "intestine" (see enteric) + -itis "inflammation."
"pertaining to the intestines," 1822, from Latinized form of Greek enterikos "intestinal," first used in this sense by Aristotle, from entera (plural; singular enteron) "intestines," from PIE *enter-, comparative of root *en "in."
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.