the butt of a/the joke
The target of a joke; the person or subject being mocked or ridiculed. My big brother loves to tease me—every time he opens his mouth, I'm the butt of the joke. No one likes being the butt of a joke, but the best thing to do is just laugh with them.
butt of a joke
the reason for or aim of a joke, especially when it is a person. Poor Fred was the butt of every joke told that evening.