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.
by potatoninja July 28, 2015
The exact opposite of a butter face. A girl who has a great face but not a good body.
I diagnosed myself with I can't believe it's not butter! I so need to start working out!
by Kaydaymuchluv81 December 2, 2011
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!
by Sionw October 14, 2018
Insult used with somebody's name, used if the person has man boobs
"Whoa, you see that guy's man boobs?"

"Yeah, I can't believe it's not butter"
is when you see an ugly person that has some how made themself look attractive through clothes, make-up or other non-plastic surgery enhancement. To test this on someone after a few cocktails (less than three) consider if you would park the love rod in the ham wallet. If yes: "I can't beleive its not butter!" - if no: see fugly.
"Wow, have you seen what Sally is wearing? She's sort of cute now."

Yeah, I know! I can't believe it's not butter!
by prosebrosproductions June 17, 2010
A girl who is so ugly you can't bare to look at her. But her face is actually not that bad.
Rob "Yo check out that huge b*tch"

Dimitri "Wow shes huge but her face isn't that bad"

Greg "Holy shit she's got an I can't believe its not butter face"
by Daga Jobin August 4, 2010
What Peter Griffin likes to say at random times, usually after other characters say "I can't believe..."
Meg: I can't believe I'm still jealous.
Lois: I can't believe I touched him.
Peter: I still can't believe it's not butter!
by Chesspiece#1 July 1, 2010