Source : Webster's Revised Unabridged Dictionary (1913)

Ruga \Ru"ga\, n.; pl. {Rug[ae]}. [L.] (Nat. Hist.)
   A wrinkle; a fold; as, the rug[ae] of the stomach.

Source : WordNet®

ruga
     n : (anatomy) a fold or wrinkle or crease; "rugae of the
         stomach"
     [also: {rugae} (pl)]