mid-13c., "timid, fearful, full of terror," from fright (n.) + -ful. The prevailing modern sense of "alarming, full of occasion for fright" is from c. 1600. Meaning "dreadful, horrible, shocking" (often hyperbolic) is attested from c. 1700; Johnson noted it as "a cant word among women for anything unpleasing." Related: Frightfully; frightfulness. Middle English also had frighty "causing fear," also "afraid" (mid-13c.).
