Def. 2 (verb) To get back into a habit. Traditionally this would refer to a good habit, but sarcastically might refer to a bad habit or addiction. Possible confusion with the phrase "on the sauce".
Def. 2 - I'd been trying to do 30 minutes of exercise every day, but my work schedule eventually got so busy I could never make it to the gym. Things have calmed down quite a bit recently though and I keep meaning to get back on the horse and go work out again.