Your mom Is fat
It's a derivative of the "Your mom" insult. The function varies slightly from the original insult as it specifies the condition the so-called "mom" is in.
Rob: Your mom Is fat.
8yo kid: *cross*
8yo kid: *cross*
your moms fat juicy mommy milkers
Your mom’s tits. 😐 What the fuck did you think I’d say?
Your moms fat juicy mommy milkers are summoning me to an early grave.