A hypothetical activity in which cows sleeping while standing up at night are tipped over by a modest exertion of force by (an often inebriated) person. This activity is believed to be physically impossible to accomplish for any reasonable large cow. Most commonly it is played as an in joke among rural people to make fun of urban folks who do not know any better and can be easily convinced that it is possible.
See Snipe Hunt.
Joe and I got plastered and went out cow-tipping. What a blast!