Roughly: “sleeve swishing” / meaning: the fear that you feel before a challenging situation
You wake up in the morning and think to yourself: Holy cow, today’s the day I have that date with that super-hot lady (or guy). Instead of being full of anticipation, you feel strangely stressed, almost paralyzed. So in the end you think to yourself, hm, actually, I have a slightly scratchy throat, maybe it’s better to cancel…
This is a classic example of the type of anxiety that Germans like to call Muffensausen. You mostly feel it in the run-up of a challenging situation. Muffe refers to the hollow cylinder that connects two pipes, and is called a “sleeve” in English. As the human digestive tract also resembles a sort of pipe system, the word Muffe became an idiomatic expression for… well, the rear end of it, so to speak. So it’s not hard to imagine what is “swishing” through your body and then through your Muffe in this metaphor.
In other words: When you have Muffensausen, you are figuratively on the verge of crapping your pants. Before that really happens, it might indeed be a better idea to cancel the date and have some drinks with your friends instead.