Old English aldormonn (Mercian), ealdormann (West Saxon) "Anglo-Saxon ruler, prince, chief; chief officer of a shire," from aldor, ealder "patriarch" (comparative of ald "old;" see old) + monn, mann "man" (from PIE root *man- (1) "man").
Presumably originally of elders of the clan or tribe, but already in Old English used for king's viceroys, regardless of age. In later Old English a more specific title, "chief magistrate of a county," having both civic and military duties. The word yielded under Canute to eorl (see earl), and after the Norman Conquest to count (n.). Having lost its specific sense, alderman was then applied to any head man; meaning "headman of a guild" (early 12c.) passed to "magistrate of a city" (c. 1200) as the guilds became identified with municipal government. Related: Aldermancy; aldermanic.