Off the wagon - Wikipedia, the free encyclopedia:
A horse-drawn wagon initiated the idiom a derivative of "off the wagon," which as of the early 1900's has meant to swear off drinking alcoholic beverages. The "wagon" in "on the wagon" refers to an icon of North America's past, the water wagon. Before roads were routinely paved, municipalities would dispatch water wagons to spray the streets in order to curb the clouds of dust that traffic would otherwise raise. Anyone who had sworn abstinence from alcohol (presumably drinking primarily water henceforth) was said to have "climbed aboard the water wagon," later shortened to "on the wagon."" To "fall off the wagon" was a logical metaphor for having failed in one's decidedness to eschew alcohol only to resume its consumption.