What is the reason for "laughing up and down" in A Dream of Red Mansions?

The 40th time of A Dream of Red Mansions: Shi Taijun's two banquets and the Grand View Garden; Jin Yuanyang Sanfangya Brand Order (Excerpt)

I saw a daughter-in-law standing there with a box, and a maid came up and opened the lid, which contained two bowls of vegetables. Li Wan took a bowl and put it on Granny Liu's desk. Xi-feng picked a bowl of pigeon eggs and put it on Mrs. Liu's desk. Grandma Liu stood up and said loudly, "Lao Liu, Lao Liu, you eat like an ox, and you don't even look up when you eat an old sow." I drummed my cheeks and was silent. At first, everyone was puzzled, and then everyone burst out laughing.

The reason is that granny Liu was cheated by Xifeng and said, "Lao Liu, Lao Liu, eats like a cow and doesn't look up after eating an old sow." A joke.

Everyone thought granny Liu was still a little silly, and they all burst out laughing.