"inflammation of the peritoneum," 1776, medical Latin, coined c. 1750 by French pathologist François-Boissier de la Croix de Sauvages (1706-1767) from Greek peritonos (from peritonaion; see peritoneum) + -itis "inflammation."
"membrane lining the abdominal cavity," early 15c., from Late Latin peritonaeum, from Greek peritonaion "abdominal membrane," literally "part stretched over," noun use of neuter of peritonaios "stretched over," from peri "around" (see peri-) + teinein "to stretch," from PIE root *ten- "to stretch." Related: Peritoneal.
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.