Basically means the same thing as thicc, someone who's not a skinny twig and has an attractive amount of meat on their bones. The only difference is that mostly ghetto southerners use this form of the phrase by adding the unnecessary 'up'.
"Hey Billie, I've been hitting up Emma a lot lately. You think she's hot?"
"She's upside down thick. Personally I'm a thigh guy but yeah I guess she is."