Dictionary.net

Vassals or tenants of the same lord, who were obliged to serve and attend him in his courts, being equal in function. These were termed "peers of fees," because holding fees of the lord, or because their business in court was to sit and judge, under their lords, of disputes arising upon fees; but, if there were too many in one lordship, the lord usually chose twelve, who had the title of peers, by way of distinction; whence, it is said, we derive our common juries and other peers. Cowell.