On the Wagon

Began life in the early 20th-century United States as "on the water-wagon" or "on the water cart," a real horse-drawn vehicle used to spray unpaved city streets with water during dry summer months to keep the dust down. Someone who'd sworn off alcohol was said to be so committed that they'd rather drink from the water-wagon's tank than touch a drink, a vivid, slightly exaggerated image of total abstinence. Over time the "water" was dropped, leaving simply "on the wagon." A genuine link to the era's active temperance movement seems likely given the timing, though it's never been definitively documented.