early 15c., "worthy of respect," from Middle French reverend, from Latin reverendus "(he who is) to be respected," gerundive of revereri "to stand in awe of, respect, honor, fear, be afraid of; revere," from re-, intensive prefix (see re-), + vereri "stand in awe of, fear, respect," from PIE *wer-e-, suffixed form of root *wer- (3) "perceive, watch out for."
As a form of address for clergymen, it is attested from late 15c.; earlier reverent (late 14c. in this sense). Abbreviation Rev. is attested from 1721, earlier Revd. (1690s). Very Reverend is used of deans, Right Reverend of bishops, Most Reverend of archbishops.