JOEL - Just as a jackrabbit looks like a rabbit but isn't (it's a hare), the term "jack-mormon" refers to someone claiming to be Mormon, but does not follow the teachings of the church. Historically the term had its beginnings at around 1838-1839. At that time the term refered to individuals, not members of the Church, who were sympathetic to the Mormon cause. Often these individuals were close friends of Mormons, uninvolved champions of religious liberty, or individuals in the process of converting to Mormonism. But, of course, the meaning has changed over the years. Now a "jack mormon" is a Mormon by birth or conversion but a backslider by nature. In Utah it can also refer to seagulls that won't eat crickets :-)
You might find more about this term in the following publication:
Church History 341 - Chapter 11
(Doctrine and Covenants course student manual)