c. 1300, rasour, "sharp-edged instrument for shaving or cutting hair," from Old French rasor, raseor "a razor" (12c.), from raser "to scrape, shave," from Medieval Latin rasare, frequentative of Latin radere (past participle rasus) "to scrape, shave" (see raze (v.)). Compare Medieval Latin rasorium.
As a verb, by 1827 as "shave with a razor," 1937 as "assault with a razor." The razor clam (1835, American English) is so called because its shell resembles an old folding straight-razor. Razor edge, figurative of sharpness or a fine surface, is by 1680s. Razor-blade is attested by 1816.