Drunk: "Not sure if I can. Seems like a good idea to try."
Sober: "I doubt I can. Reason dictates that I don't, in order to avoid damaging myself."
Truth is, most people who are not overweight (and don't have a back issue, or the like) are able to do a backflip with about 3 minutes of training. Most also need to be drunk to be talked into it.
I can do a frontflip the way Bruce Campbell did in Evil Dead 2, by grabbing the back of my shirt collar and tugging. I can't do a regular one though, and I've never even thought of trying a backflip.