Jack Mormon equivalents in other religions are "Christmas and Easter Christians" and "Yom Kippur Jews".
2. A person born in a family with Mormon roots, but who does not live all the precepts of the faith.
3. A person not unfriendly to Mormons, or who might even believe some or all Mormon tenets, but who - while not being adversarial - does not participate in the sacraments of the faith, or abstain from proscribed actions (such as smoking, or drinking coffee, tea, or alcoholic drinks).
4. (Archaic) A person with many Mormon acquaintances and friends and/or who sympathizes with them and their beliefs, but chooses not to affiliate with the church.
"She likes to go to Relief Society meetings, but she just can't give up her doubts about the Book of Mormon or her morning cup of coffee. She's a confirmed jack Mormon."
From experience in Salt Lake City (HQ of Mormonism), this person(typical Jack-Mormon) cannot wait to escape the confines of Utah over the border to Nevada, often going to the small city of Wendover being the nearest to get to, to partake in prostitution, alcohol and gambling.
2: I thought she was Mormon!
1: She's a jack mormon.