peritoneum (n.)

"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.