When a drunk quits drinking, it is said that s/he’s on the wagon. What wagon? To what does this expression refer?
Seconding the question
WAG: In frontier days, if you were drunk, you fell off your (covered) wagon. Staying on the wagon, in effect, means staying sober.
Originally “On the water wagon”. Coined by the temperance movement, to signify that someone had pledged to abstain from drinking:
ETA:
Quinion doesn’t say it was coined by them, just that that’s where it came from. Other sources say they coined it.
It was originally “on the water-wagon” (and before that, “on the water-cart”).
Water carts carried water to damp down dusty streets in olden times. If you’d given up drinking liquor your other option might be to sup from the water cart.
Edit… Simulpost (almost) 
I gotta admit…I could never quite remember whether being **on **the wagon was good thing or a bag thing…kinda like whether being up shit creek without a paddle was good thing or bad thing either…
Sheesh, bil, even I know that:
Being on the wagon, GOOD
Up shit creek: BAD
But if you’re UP shit creek, you can always drift back down, no paddle needed. (Shit notwithstanding, that was added later for effect.)
Creeks get bigger as you go down them, so I don’t see how going down shit creek is making things any better 
I guess if I had to get all grammary on it, “on or in shit creek” would be a bit more cut and dried to me.
Just saying.
I’ve heard the explanation that it was reference to condemned men being allowed a last drink in a pub on the way to their execution. Once they had finished it they got on the wagon to go to the gallows, hence the once you were “on the wagon” you have had your last ever drink.
Never heard a shred of evidence to support it (or for that matter a shred of evidence to support the idea that condemned men were offered a last drink, it would after all seem the obvious time attempt an escape, especially if you had accomplices in the pub in question).