A lisp
It’s when you try to say 's' and it comes out like 'th' and people think you're an idiot.
I tried to say 'silly' and it came out 'thilly' and the whole class laughed at me.
He told me he had a lisp and I said 'thith' and I got called a fool.
She tried to say 'soda' and it came out 'thoda' and the teacher said she was a moron.