When you 'fall off the wagon', you go back to drinking alcohol in large quantities after having abstained from it for a while. Nowadays, the expression is used to refer to the resumption of any bad activity — drugs, smoking, overeating, etc.

What does the phrase on the wagon mean?

Abstaining from drinking alcoholic beverages, as in Don't offer her wine; she's on the wagon. This expression is a shortening of on the water wagon, referring to the horse-drawn water car once used to spray dirt roads to keep down the dust. Its present meaning dates from about 1900.

Is it on or off the wagon?

What's the meaning of the phrase 'On the wagon'?

'On the wagon' - abstaining from alcohol. 'Off the wagon' - returned to drinking after an attempt to give it up.