be off the wagon

1. To be drinking alcohol again after a period of abstinence. Usually said of recovering alcoholics. Did you smell the whiskey on his breath? I think Tom is off the wagon. A: "Wow, Dad's a real mean son of a gun again." B: "I know. He must be off the wagon." A: "It's just easier to socialize when I'm off the wagon, OK?" B: "But to the total detriment of your health, though."

2. By extension, to be engaged again in some behavior that one previously ceased, typically something that is detrimental in some way. I gave up smoking nearly three years ago, but I've been off the wagon ever since Jeff's bachelor party. A: "You saw him at the club last night? I thought he'd stopped partying once he had kids." B: "Well, he must be off the wagon then." I did really well with not biting my nails for a while, but I'm off the wagon these days.