A butter substitute made of vegetal oils and frequently used as a spread. Sometimes its name is used as a humorous/sarcastic reference to a sub-par substitute to something.
<Eats a piece of toast>
- Just a moment, is it butter?
- No, it is "I can't believe it's not butter!"
- Well, *I* can believe it is not butter.
A popular phrase to say when you are currently in denial that what you have been served on your warm toast is not butter, but rather, margarine.
The phrase became especially after a brand of margarine used it as their name, to show just how their margarine was more similar to the real thing, however, now it's pretty normal that margarine tastes like butter.
Person A: This toast is so tasty. What is the brand of butter you use?
Person B: That's not butter, that's margarine.
Person A: What? I can't believe it's not butter!