"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.
word-forming element meaning "excessive or irrational fear, horror, or aversion," from Latin -phobia and directly from Greek -phobia "panic fear of," from phobos "fear" (see phobia). In widespread popular use with native words from c. 1800. In psychology, "an abnormal or irrational fear." Related: -phobic.
mid-15c., "causing fear," from Old French formidable (15c.), from Latin formidabilis "causing fear, terrible," from formidare "to fear," from formido "fearfulness, fear, terror, dread." Sense has softened somewhat over time, in the direction of "so great (in strength, size, etc.) as to discourage effort." Related: Formidably.