A person who admonishes; an admonisher; specifically (chiefly with capital initial) any of the Puritans associated with the Admonition to the Parliament of 1572, condemning the episcopal government and ceremonies of the Church of England.
The authors of the Admonition were the Oxford graduates John Field and Thomas Wilcox, who were supported by Thomas Cartwright, a former Chair of Divinity at Cambridge.
Late 16th century; earliest use found in Richard Bancroft (bap. 1544, d. 1610), archbishop of Canterbury. From admonition + -er.