This idiom describes relapsing into a bad habit or addiction, particularly after a period of abstinence or improvement, such as resuming drinking after sobriety. It suggests a failure to maintain resolve, often with a sense of regret or struggle, and is used in contexts of addiction, dieting, or personal goals. The phrase carries a sympathetic or judgmental tone, reflecting the cultural tension between self-discipline and human frailty, and is often associated with recovery narratives or moral lapses.

The phrase originated in late 19th-century America, tied to the temperance movement, where ‘on the wagon’ referred to staying sober, possibly from water wagons used by teetotalers. ‘Falling off the wagon’ emerged to describe relapse, with an early use in a 1901 *Los Angeles Times* article: ‘He fell off the wagon after months of sobriety.’ The idiom gained traction during Prohibition (1920–1933), reflecting widespread struggles with alcohol, as seen in Sinclair Lewis’ *Elmer Gantry* (1927). Its use expanded in the 20th century to include other habits, particularly through Alcoholics Anonymous and recovery culture, ensuring its spread in American and global English.