chaplain (n.)

mid-14c., "minister of a chapel," from Old French chapelein "clergyman" (Modern French chapelain), from Medieval Latin cappellanus "clergyman," originally "custodian of St. Martin's cloak" (see chapel).

It replaced late Old English capellane (from the same Medieval Latin source), the sense of which was "clergyman who conducts private religious services," originally in great households; this sense continued in chaplain and later was extended to clergymen in military regiments, prisons, etc.