off the wagon

Etymology
Originally off the water wagon or off the water cart, referring to carts used to hose down dusty roads:  see the 1901 quotation below. The suggestion is that a person who is “on the wagon” is drinking water rather than alcoholic beverages. The term may have been used by the early 20th-century ; for instance, (1874 – ), the superintendent of the, is said to have made the following remark about Prohibition: “Be a good sport about it. No more falling off the water wagon. Uncle Sam will help you keep your pledge.”

Prepositional phrase

 * 1)  No longer maintaining a program of self-improvement or abstinence from an undesirable habit, especially drinking alcohol.