1
general::
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. The antonym off the wagon, used for a resumption of drinking, dates from the same period. B.J. Taylor used it in Extra Dry (1906): "It is better to have been on and off the wagon than never to have been on at all."
American Heritage Idioms
2
general::
Phrase(s): on the wagon
Fig. not drinking alcohol. • No, I don’t care for a cocktail. I’m on the wagon. • Bob’s old drinking buddies complained that he was no fun when he went on the wagon.
McGrawhill's American Idioms And Phrasal Verbs