While at the movies, you take a tub of popcorn and make a hole at the bottom. Then you stick your dick through it and while watching the movie, your date sticks her hand in it as if to grab popcorn. This will look that way to the movie-goers around you. In reality, she jerks you off inside the tub until you cum all over the popcorn and then she eats it. Hence: butter the popcorn.
Sean: "I'm taking Melissa to the movies tonight. You know how easy she is, so I know we are going to butter the popcorn. I'll text you tomorrow and tell you all about it in complete detail. I'll even save you some of the popcorn."
Matt: "Dude, isn't that kind of gay?"
Sean: "Hey, I won't tell if you don't."