“Where does the term ‘Jack-Mormon’ come from?” Ensign, Mar. 1974, 25
In Old English the term “Jack” was sometimes placed before a noun denoting a person, a trade, or a group, and was often applied contemptuously. As far back as the 16th century such terms were used as “Jack-gentleman,” who was a man of low birth pretending to be a gentleman; “Jack-meddler,” a busybody; “Jack-priest,” a person who was not a priest but acted like one; and even “Jack-sauce”—a sauce that one would presume to be the real thing but that lacked some essential ingredient.
So far as we are able to tell, the term “Jack-Mormon” was first used by the Warsaw Signal when contemptuously referring to Illinois non-Mormons who were friendly with Latter-day Saints and sympathetic to their rights. Non-Mormon Sheriff Backenstos of Nauvoo was called a “Jack-Mormon” because he opposed lawless elements who were trying to drive the Saints out of Hancock County. Sometimes manifestos published by anti-Mormons explicitly mentioned this group, asserting that both the Mormons and the “Jack-Mormons” (persons who purchased property from the fleeing Mormons) needed to be exterminated. This use of “Jack-Mormon” continued throughout the latter half of the 19th century, when a number of speeches and newspaper articles used the term in that sense.
In the early years of this century, particularly after World War I, the term came to have another meaning. Once anti-Mormon literature and agitation began to disappear, it was no longer possible to refer to all those who were friendly to Mormons as “Jack-Mormons.”
In this century Latter-day Saints began to use the term in reference to persons who were only nominal members of the Church; that is, members who were not valiant in conforming to Church standards. Just as the early Hancock County “Jack-Mormons” were friendly to the faith but did not feel any compunction to live Church standards, pay tithing, and heed the advice of the Prophet, so there were Latter-day Saints who, although members, consistently violated the standards of the Church. These came to be called “Jack-Mormons.”