"irrational fear, horror, or aversion; fear of an imaginary evil or undue fear of a real one," 1786, perhaps based on a similar use in French, abstracted from compounds in -phobia , the word-forming element from Greek phobos "fear, panic fear, terror, outward show of fear; object of fear or terror," originally "flight" (still the only sense in Homer), but it became the common word for "fear" via the notion of "panic flight" (compare phobein "put to flight; frighten"), from PIE root *bhegw- "to run" (source also of Lithuanian bėgu, bėgti "to flee;" Old Church Slavonic begu "flight," bezati "to flee, run;" Old Norse bekkr "a stream").
The psychological sense of "an abnormal or irrational fear" is attested by 1895. Hence also Phobos as the name of the inner satellite of Mars (discovered 1877) and named for Phobos , the personification of fear, in mythology a companion of Ares.
"morbid dread of a current of air," 1785; see aero- + phobia.
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."