A fork of Rural Dictionary
When a cleavage looks kind of perky and happy but also a little on the sad any unperky side at the same time.
"Have you seen Emily's breasts? Her tits are like "Wow!" but also kind of "Meh." at the same time... I feel like her cleavage is somehow smiling at me--but also frowning at me! It's like she's got that whole Mona Lisa Cleavage thing going on. You know, like auntie Sara."
The notion that while there isn't necessarily something wrong with someone/something, the two elements aren't a match.
"Dude, why did y'all break up?" "Classic Mona Lisa Principle. She was beautiful, mysterious, and intriguing, but (pointing to my brain) didn't match a damn thing in my house."