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 cerain foods, masturbation, 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.
by not_really_me March 15, 2004
Originally a phrase used when an alcoholic resumes drinking. The wagon is presumably the high school or college band wagon. In the 1950s, schools used to hold pep rallies for football teams. Often the music band would be on a wagon and the enthusiastic students would hop on the wagon. Alcohol was not allowed on the wagon. The phrase "back on the wagon" refers to one who quits drinking and hops on the band wagon, with a positive spirit, supporting the team and school. In other words, one who is happy to not drink. Relapsing into alcoholism is often sudden and unintentional, hence the description of falling off the wagon.
I guess John fell off the wagon last night, he was hammered.
by ScottDMF October 9, 2015
A phrase used to indicate that a romantic or sexual relationship has ended, or is, as when the wheels do fall off a wagon, kaput. Sometimes heard as "wheels fall off the wagon" or "wheels falling off the wagon"
"She found out he was also shagging her brother and so the wheels fell off the wagon then."

"Her incessant screwing around on him made the wheels fall of the wagon, and he dumped her."

"If he keeps that up, I can see the wheels falling of the wagon soon."
by Alexandra July 29, 2004