Solid butter

When someone is so fat that it's hard to penetrate their ass/pussy, they are known as "solid butter" because they're too thick to spread.

Person 1: Ayy man I heard you fucked that fat chick last night. How was she?

Person 2: She was no good. That bitch was a solid butter.

Person 1: You mean she's fat from eating lots of butter?

Person 2: No, she's a solid butter because she's too thick to spread.