Though the phrase "to fall off the wagon" can allude to any number of vices, it usually refers to a person's failed attempt to give up alcohol. After doing a bit of research, we uncovered the origin of this temperate expression.
Basically, saying that a person was "on the wagon" was shorthand for "they would sooner climb aboard a water cart to quench their thirst" than have a drop of liquor.
1. fell off the wagon
When one resumes an addictive/compulsive behavior that they are trying to control. Originally referred only to drinking, but is now used in reference to all sorts
of things including drugs, smoking, overeating, eating ceratin foods, casual sex, pornography...
Joe hadn't had a drink in 10 years, but when his wife of 30 years left him, he fell off the wagon.