early 15c., also reverent, "worthy of deep respect, worthy to be revered" due to age, character, etc., from Old French reverent, reverend and directly 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-, here perhaps an intensive prefix (see re-), + vereri "stand in awe of, fear, respect" (from PIE 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). Prefixed to names by 1640s. Abbreviation Rev. is attested from 1721, earlier Revd. (1690s). Very Reverend is used of deans, Right Reverend of bishops, Most Reverend of archbishops.
"clergyman," c. 1500, from reverend (adj.). Used as a courteous or respectful address from late 15c.