Jack-mormon Definition

noun

A member of the Church of Jesus Christ of Latter-day Saints who believes in the premises of the faith but does not adhere to the church's standards of conduct, such as its prohibition of the consumption of alcohol, coffee, and tobacco.

Wiktionary

Origin of Jack-mormon

The term was first used in the 1830s by Mormons in Missouri to describe non-Mormons who were friendly to Mormons living in their community. The meaning subsequently shifted.

From Wiktionary