-phobia

"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 fear of heights," 1887, medical Latin, from Greek akros "at the end, topmost" (from PIE root *ak- "be sharp, rise (out) to a point, pierce") + -phobia "fear." Coined by Italian physician Dr. Andrea Verga in a paper describing the condition, from which Verga himself suffered.

In this paper, read somewhat over a year ago at the congress of alienists at Pavia, the author makes confession of his own extreme dread of high places. Though fearless of the contagion of cholera, he has palpitations on mounting a step-ladder, finds it unpleasant to ride on the top of a coach or to look out of even a first-story window, and has never used an elevator. [abstract of Verga's report in American Journal of Psychology, November 1888]