1. A nominal, or non-practicing, Mormon
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.
"He's a jack Mormon, but if anyone's in trouble he's the first one there to help"
"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."