It means you can't help laughing. The tortoise couldn't help laughing after graduation, which is actually a two-part allegorical saying, meaning that the tortoise doesn't live in school, and the tortoise means the tortoise. "I can't hold it (laugh)" homophonic terrier. Two-part allegorical saying is composed of two parts: the former part acts as an introduction, like a riddle, and the latter part acts as a background, like a riddle, which is very natural and appropriate. In a certain language environment, you can understand and guess the original intention by saying the first half sentence and "resting" the second half sentence, so it is called two-part allegorical saying.