It's the bunk.
4 seconds sound clip from the Christmas in July (1944) classic radio play.
You can hear this line at 00:07:16 in the radio play.
Quote context
[...]
- Now that's simple enough, isn't it? Coffee makes you sleep!
- Well, he says it makes you sleep. He's a Viennese doctor.
- Coffee keeps you awake. That's a well known fact! Why are you so pig headed about it?
- So I say if you don't sleep at night it isn't the coffee it's the bunk. Do you get it?
- You guess so, what's it mean?
- Yes. But do you get the play on words?
- Oh Jimmy, you don't need a play on words.
- Don't you understand? Don't you understand it's funny?
- Ooh!
[...]