water wagon
/ˈwɔːtə ˌwaɡ(ə)n/noun
1A cart, usually a barrel or tank on wheels, carrying water; specifically one used to water the streets by sprinkling water from small holes as it goes along; = water cart. Chiefly US in later use.
2 Informal (originally US ). In various expressions referring to abstinence from alcoholic drink, especially in on the water wagon.
Origin
Mid 18th century; earliest use found in Daily Advertiser. From water + wagon.