Middle English fere, from Old English fær "calamity, sudden danger, peril, sudden attack," from Proto-Germanic *feraz "danger" (source also of Old Saxon far "ambush," Old Norse far "harm, distress, deception," Dutch gevaar, German Gefahr "danger"), from PIE *pēr-, a lengthened form of the verbal root *per- (3) "to try, risk."
The sense of "state of being afraid, uneasiness caused by possible danger" developed by late 12c. Some Old English words for "fear" as we now use it were fyrhto, fyrhto; as a verb, ondrædan. Meaning "feeling of dread and reverence for God" is from c. 1400. To put the fear of God (into someone) "intimidate, cause to cower" is by 1888, from the common religious phrase; the extended use was often at first in colonial contexts:
Thus then we seek to put "the fear of God" into the natives at the point of the bayonet, and excuse ourselves for the bloody work on the plea of the benefits which we intend to confer afterwards. [Felix Adler, "The Religion of Duty," 1905]
Originally transitive in English; long obsolete in this sense but somewhat revived in digital gaming via "fear" spells, which matches the old sense "drive away by fear," attested early 15c. Meaning "feel fear" is late 14c. Related: Feared; fearing.
Proto-Indo-European root meaning "to try, risk," an extended sense from root *per- (1) "forward," via the notion of "to lead across, press forward."
It is the hypothetical source of/evidence for its existence is provided by: Latin experiri "to try," periculum "trial, risk, danger;" Greek peira "trial, attempt, experience," empeiros "experienced;" Old Irish aire "vigilance;" Old English fær "calamity, sudden danger, peril, sudden attack," German Gefahr "danger," Gothic ferja "watcher.