What does this joke mean? I don't quite understand.

Two male and female nurses are together. The old nurse said to the little nurse: Today, a patient has a real personality, and that thing is still tattooed! The little nurse asked, What did you write? The old nurse said: first class! The next day, the patient came again and the little nurse saw it. The little nurse said to the old nurse after reading it, you are wrong. It is clearly written that a river flows eastward, seven words! This is a joke. As for the taste, take your time.