Twilight and Rainbow have been drinking buddies for quite some time now. Every Friday, they go out to a bar for a couple of hours and forget the troubles of the day.
But on this particular Friday, Rainbow's being even more of an ass than usual. So Twilight does what any sane pony would have done in this situation:
Smacks her friend in the face as hard as she can with a dictionary.
Rated Teen for language and a very drunk Rainbow Dash.