Green’s Dictionary of Slang

1. a non-Mormon who sympathizes with the Mormons; thus jack-Mormonism n.

1846		Warsaw (IL) Signal 6 Feb. 3/1: A certain Jack-mormon of Hancock county [...] is in the habit of shaving the hair off his forehead, in order to give it an intellectual appearance [DA].
1870		J.H. Beadle Life in Utah 130: It was privately announced that the wolves to be hunted were the Mormons, and Jack Mormons* [*A slang name applied to Gentiles who favor the Mormons]. [Ibid.] 197: From 1850 to 1862, ‘jack-Mormonism’ ruled at Washington.
1881		Cornishman 10 Feb. 7/6: Any tender-0fiooted pilgrim from Missouri, Jack Mormon from Utah [...] can explode giant power in the crevice of a rock.
1886		Salt Lake Herald (UT) 23 Feb. 7/2: He will have it thrown into his teeth that he is a Jack Mormon.
1904		Goodwin’s Wkly (UT) 22 Sept. 4/3: These political mendicants [...] insist that being always Jack-Mormons, they ought at least be permitted to gorge themselves at the church swill barrel.
1910		Salt Lake Trib. (UT) 11 Nov. 4/4: Every Jack Mormon is now splitting his lungs in the yell for peace in Utah.
1913		(con. 1870) Jocknick Early Days on the Western Slope of Colorado 10: An impecunious ‘Jack Mormon’ possessing but one wife.
1927		C.M. Russell Trails Plowed Under 69: This Murphy ain’t no real Mormon. He’s what we’d call a jack-Mormon.
1947		Time 21 July 21/1: The number of backsliding ‘jack-Mormons’ is increasing.
1955		E. Hemingway letter 14 Nov. in Baker Sel. Letters (1981) 850: The most comical thing I remember about the late De Voto (a jack Mormon).
a.1985		‘The Open Book’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 112: There’s that whistle-prick out there in Utah; / he was sired by old Brigham Young [...] Often called the crying Jack Mormon.

2. attrib. use of sense 1.

1869		Glasgow Herald 25 Dec. 3/5: General Deeming, the Jack-Mormon sheriff.
a.1946		‘The Open Book’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 116: There’s a so-called cowhand in Utah, / Who thinks he’s the salt from Salt Lake; / He’s only small change from that locoweed range, / With less brains than to make his head ache. / He thinks he’s a wonderful twister, / This boob with the jack-mormon brand.

3. an apostate Mormon.

1967

E. Shepard Doom Pussy 58: Nails was a Jack Mormon of Welsh and Italian descent.