Someone who changes their alleged fundamental belief(s) or allegiance(s) in keeping with the popular views of the time, so as to gain or maintain a favorable position or advantage. (An allusion to Simon Aleyn, a 16th-century vicar in the town of Bray, Berkshire, who changed his religious doctrine between Roman Catholicism and Protestantism to that of the ruling monarch.) Though the statesman always remained popular in the polls, many politicians felt him to be a Vicar of Bray, changing the tune of his rhetoric to meet whatever fancy the public demanded at the time.