哄hōng堂táng大dà笑xiào

Literal: uproar in the hall, great laughter
Figurative: A whole roomful of people bursting out laughing at once, be it a class, an audience, or a meeting; general, spontaneous laughter.

Where does 哄堂大笑 come from?

It goes back to a Tang-dynasty custom recorded by Zhao Lin in his Yinhua lu. In the imperial Censorate, when officials ate together in the official hall, they were expected to stay silent and not laugh, and anyone who did risked a penalty. But if the official presiding over the table laughed, everyone laughed, and this was called 烘堂 (now written 哄堂), "setting the hall in an uproar"; in that case no one was punished. Over time the word came to mean any collective burst of laughter. Today it describes a whole class, audience, or meeting laughing together, with no trace of the old ceremony.

Source (出处): 赵璘《因话录》

How is 哄堂大笑 used? Two examples

年nián夜yè饭fàn上shàng，小xiǎo侄zhí子zi学xué爷yé爷ye打dǎ呼hū噜lu，满mǎn屋wū子zi的de人rén哄hōng堂táng大dà笑xiào。
At Lunar New Year's Eve dinner, my little nephew imitated Grandpa snoring, and the whole room burst out laughing.
开kāi会huì时shí屏píng幕mù上shàng突tū然rán跳tiào出chū老lǎo板bǎn家jiā猫māo的de照zhào片piàn，会huì议yì室shì里lǐ哄hōng堂táng大dà笑xiào。
In the middle of a meeting, a photo of the boss's cat suddenly popped up on the screen, and the whole room cracked up.

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