rarefy v. + Add to list US //ˈɹɛɹəfaɪ// rar·efy SynonymsEtymology Synonyms attenuatesublimatesubtilizeOriginLATrarefacere→OFRrarefier→MErarefien→ENrarefy From Middle English rarefien, from Old French rarefier (French raréfier), from Latin rarefacere (“make rare”). By surface analysis, rare + -fy.