per ‧ i ‧ to ‧ ni ‧ tis /ˌperətəˈnaɪtəs, ˌperɪtəˈnaɪtəs $ -tnˈaɪ-/ BrE AmE noun [uncountable] technical
[ Date: 1700-1800 ; Origin: peritoneum 'inside wall of the abdomen' (16-21 centuries) (from Late Latin , from Greek ) + -itis ]
a serious condition in which the inside wall of someone’s ↑ abdomen (=part around and below your stomach) becomes infected and painful