The saying originates with the English Civil War. It was the custom for Royalist prisoners to be dispatched to the Parliamentarian garrison at Coventry for internment. Because Royalist prisoners were 'the other side' it was customary for the soliders, and the townsfolk who came into contact with these prisoners, to ignore them. This gives rise to the popular phrase 'being sent to Conventry' meaning to be ignored or excluded from a group.