Das ist eher wie "abgedroschener Spruch". Wenn der 表哥 aber dem Gesprächspartner oder in der Story mit Namen bekannt ist kann es auch ein richtiges Zitat sein. Man beleidigt ja nicht seine eigenen Verwandten.
表哥 is your mother's sister's son that is older than you. 名言 means ”famous speech“。 This has nothing to do with "an old joke" or "ein abgedroschener Witz". In China, we don't have such fixed expression, it has very concrete meaning, namely, literal meaning.
You can try this:
The famous speech from/of my cousin。
But notice from "cousin" people cannot where this guy come from, from your father side or mother side, and people have no clue if he is older or younger than you. Did you see the beauty of Chinese, just with two characters "表哥“,a lot of information is expressed. Cooooooooool!
And you should avoid using "saying", because IMHO, saying is something life that most people believe is wise and true. But what your or my cousin said is only his philology or opinion, it could be completely offensive or stupid.