Origin: 1425–75; late Middle English < Middle French < Late Latin decad- (stem of decas) < Greek dekad- (stem of dekás) group of ten, equivalent to dék(a) ten + -ad--ade2
mid-15c., "ten parts" (of anything; originally in ref. to the books of Livy), from M.Fr. decade, from L.L. decadem (nom. decas), from Gk. dekas (acc. dekada) "group of ten." Meaning "ten years" is 1590s in English.