once in the Warring States Period (zhàn guó 战国,481 - 386BC), the State of Jin (jìn guó 晋国) was at war with the State of Chu (chǔ guó 楚国). Duke Gong of Chu (chǔ gòng wáng 楚共王) stood on a high platform built on chariot and watched the movements of the Jin army. After watching for a while,he said,"It's quite noisy over there and cloud of dust has been stirred up."  His aide answered, "The enemy are filling up wells and destroying their cooking stoves. They are preparring to fight."

This idiom is now used to mean a lot of commotion over hearsay.

战国时代，晋国和楚国交战。楚共王站在架有高台的战车上观察晋军的动静。看了一会儿，楚王说：“那天人声十分喧嚣，而且尘土都飞扬起来了！”部下回答说：“这是他们在填井平灶，摆开阵势要作战啦。”

现在，“甚嚣尘上”这个成语被用来形容军中忙乱的状态，也指消息普遍流传，议论纷纷。现多用来指敌方狂妄叫嚣。

甚嚣尘上 (shèn xiāo chén shàng)
【翻译】Making a Great Clamor.
【释义】甚：很；嚣：喧嚷。人声喧嚷，尘土飞扬。原形容军中正忙于准备的状态。后来形容消息
            普遍，议论纷纷。现多指反动言论十分嚣张。
【例句】小明考试作弊的传闻一度甚嚣尘上，引起了学校老师的高度关注。
【近义词】鸮鸣鼠暴、满城风雨
【反义词】偃旗息鼓、风平浪静