The entire quote, after young Sheldon is awakened from his binary meaning to the universe dream, Jim Parsons speaks the words:
"I was once visited by Nine when I had chickenpox. He was a lot less funny than he thought he was."
I think "visited by Nine" and "a lot less funny" probably references something because it's so specific, but I have no idea what that's referencing and neither Google nor Urban Dictionary was any help.