One of the seniors or elders. Now applied to a class of magistrates in corporate towns. In London an alderman is the chief magistrate in a ward appointed by election. There are also aldermen of the Country Council.
A turkey is called an alderman, both from its presence in aldermanic feast, and also because of its red and purple colours about the head and neck, which make it a sort of poultry alderman.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894