English word frightful comes from Old English forhtful

Detailed word origin of frightful

Dictionary entry	Language	Definition
forhtful	Old English (ang)
frightful	Middle English (enm)
frightful	English (eng)	(hyperbolic) Unpleasant, dreadful, awful (also used as an intensifier).. (obsolete) Full of fright, whether. Afraid, frightened.. Full of something causing fright, whether. Genuinely horrific, awful, or alarming.. Timid, fearful, easily frightened.