Alan Hagler

(male) Of Celtic origin and uncertain derivation (possibly a diminutive of a word meaning ‘rock’). It was introduced into England by Breton followers of William the Conqueror, most notably Alan, Earl of Brittany, who was rewarded for his services with vast estates in the newly conquered kingdom. In Britain the variants Allan and Allen are considerably less frequent, and generally represent transferred uses of surname forms, whereas in America all three forms of the name are approximately equally common. See also Alun.

German (also Hägler): topographic name for someone who lived by a hedge or enclosure, Middle High German hac, + the suffix -er denoting an inhabitant (compare Doerfler), or a habitational name from a place named with this word.