satellite of Mars, discovered in 1877, named for Greek deimos, literally "fear, terror," also, as Deimos, the personification of such, regarded as a son of Ares, twin brother of Phobos"fear, panic, flight" (for which see phobia). Greek deimos is from PIE *duei- "fear," source also of Sanskrit dvesti "hate," Avestan duuaetha "threat," and possibly Latin dirus "fearful."
From c. 1500 as "fear so great as to overwhelm the mind." Meaning "quality of causing dread" is attested from 1520s. Sense of "a person fancied as a source of terror" (often with deliberate exaggeration, as of a naughty child) is recorded from 1883. Terror bombing first recorded 1941, with reference to German air attack on Rotterdam. Terror-stricken is from 1831. The Reign of Terror in French history (March 1793-July 1794) was the period when the nation was ruled by a faction whose leaders made policy of killing by execution anyone deemed an impediment to their measures; so called in English from 1801. Old English words for "terror" included broga and egesa.
"to frighten," Middle English, from Old English fyrhtan "to terrify, fill with fear," from the source of fright (n.). Old English had also forhtian "be afraid, become full of fear, tremble," but the primary sense of the verb in Middle English was "to make afraid."