"irrational fear, horror, aversion," 1786, perhaps on model of similar use in French, abstracted from compounds in -phobia, from Greek -phobia, from 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, fright" (compare phobein "put to flight, frighten"), from PIE root *bhegw- "to run" (cognates: Lithuanian begu "to flee;" Old Church Slavonic begu "flight," bezati "to flee, run;" Old Norse bekkr "a stream"). Psychological sense attested by 1895.