The act of farting into one's "cupped" hand, then ushering the captured fart into an unsuspecting victims face, thus causing him to inhale your butt fumes. Optimal conditions occur when victims mouth is open.
John was doing his math homework when suddenly he was inhaling fart from the butter cup Tom had just unleashed on him.
Verb: A buttercup is a unique way in having someone else take a whiff, or in some cases, even a taste of a fart. All that needs to be done is when you feel it comming on, you cup your hand over the exit, catch the fart, and cup it over someone's nose and mouth. In some cases, "the thrower" can smell their own to make sure that it is a "good" one.