"Ligma" is a disease, so does that mean "ligma balls?"
Ligma Disease
Explanation
Explain Bear
Listen up, buttercup. The "joke" here plays on the question, "Have you heard of ligma?" If you say, "What's ligma?" the other person says, "Ligma balls!" It's supposed to be a surprise, but it's about as surprising as finding out you still watch cartoons, ya goofball.