rarefy (v.)youdaoicibaDictYouDict[rarefy 词源字典]
late 14c., from Old French rarefier (14c.) and directly from Medieval Latin rarificare, from Latin rarefacere "make rare," from rarus "rare, thin" (see rare (adj.1)) + facere "to make" (see factitious). Related: Rarefied.[rarefy etymology, rarefy origin, 英语词源]
subtile (adj.)youdaoicibaDictYouDict
late 14c., "clever, dexterous, crafty; not dense, thin, rarefied," from Old French subtil (14c.), a learned Latinized reformation of earlier sotil (12c.), source of subtle (q.v.). Still used in some Bible translations in Gen. iii:1, and it survived after 17c. as a parallel formation to subtle in some material senses ("fine, delicate, thin").