The act of drinking someone else's beverage without letting your mouth come into contact with the can, I.E. holding the can above your mouth and pouring it in. The idea of sky drinking comes from the unpleasant prospect of having someone else's saliva on your drink can, which won't cause any harm, but still isn't very nice to think about.
"Hey Will, let's have some of that beer."
"Alright, but sky drink it, I don't want your germs on my can."