Why do I need to know “mein Schwein pfeift”?

Because it’s not only an amusing way to express your disbelief in something, but will also help you impress your German friends with your knowledge of folksy idioms. 

What does it mean?

Quite literally, “Ich glaub’ mein Schwein pfeift” means “I believe my pig is piping”. As you might imagine, a piping pig is a rather unlikely scenario – and that’s exactly what the phrase implies.

You can use it if someone tells you an unbelievable anecdote or does something that really surprises you. For example, if your fitness-averse friend tells you they have signed up to compete in a marathon, you could say: “You’re running a marathon? I think my pig is piping!” (“Du läufst einen Marathon? Ich glaub’ mein Schwein pfeift!”)

There isn’t a direct English equivalent for the phrase, but “when pigs fly” is a fairly similar way to show your scepticism about something. Otherwise, an expression like “I can’t believe my ears!” would have the same effect. 

Where does it come from?

The phrase is believed to date back to Berlin slang in the 1920s. Rather than being a farming metaphor, the Schwein in this case refers to the bulbous, round kettles that look a bit like a piggy bank and whistle when the water boils.

To interrupt unpleasant conversations or show that they were astonished by something, people would claim that their pig was whistling (or piping). This was a jokey way of expressing their desire to remove themselves from the conversation, go into the kitchen and have a sit down. 

Over time, the phrase has taken hold as a common way to express surprise.  

Use it like this:

Bist du wirklich schon mit deinen Hausaufgaben fertig? Ich glaub’, mein Schwein pfeift!

Have your really finished your homework already? I think my pig is piping!

Ich glaube, mein Schwein pfeift. Hat er das wirklich zu dir gesagt?

I think my pig is whistling. Did he really say that to you?