The Collaborative International Dictionary
Ruga \Ru"ga\, n.; pl. Rug[ae]. [L.] (Nat. Hist.) A wrinkle; a fold; as, the rug[ae] of the stomach.
Douglas Harper's Etymology Dictionary
plural of ruga (1775), from Latin ruga "a wrinkle in the face," from PIE *rug-, variant of *reue- (2) "to smash, knock down, tear up, dig up, uproot" (see rough (adj.)).
n. (plural of ruga English)
n. (anatomy) a fold or wrinkle or crease; "rugae of the stomach"
[also: rugae (pl)]