Off the alcohol or some type of liquor. Often used in the late nights and after or at parties. Off the sippy cup is the mild version of being drunk rather than being absolutely plastered.
Going along with a stupid idea simply because fellow peers are rather than one thinking for his or herself. The term originates from when Jim Jones of Guyana lured a thousand people of his satanic cult into drinking cyanide-poisoned Flavor-Aid in 1978.
Dumbass: You wanna go play chicken?
Wise person: Why would I wanna go do something so dangerously stupid?
Dumbass: Because it's fun and all the cool people are doing it.
Wise person: All the 'cool' people are idiots! Quit sipping the Kool-Aid before you get yourself killed.
Dumbass: Hmmmm.....No thanks! My social status depends on it. I'm out! Later loser!