Stool of Repentance
A low stool placed in front of the pulpit in Scotland, on which persons who had incurred an ecclesiastical censure were placed during divine service. When the service was over the “penitent” had to stand on the stool and receive the minister's rebuke. Even in the present century this method of rebuke has been repeated.
“Colonel Knox tried to take advantage of a merely formal proceeding to set Mr. Gladstone on the stool of repentance.” —The Times.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894