Английский Этимологический словарь - rare (1)

- "unusual," 14c., "few in number and widely separated," from O.Fr. rere "sparse," from L. rarus "thinly sown, having a loose texture," hence, "unusual" (1542). Rarity is early 15c., from M.Fr. raritй, from L. raritas "thinness, fewness," from rarus.