Muffled Thunder Rolling" is not an idiom, but just an ordinary four-letter word. It means that the low sound of thunder keeps reflecting back and forth between the earth and the clouds, creating a rumbling and rolling sensation.

The term "muffled thunder rolling" comes from the text "Tide Watching" in the first book of the fourth grade of the Humanist version of the language, written by Zhao Zongcheng and Zhu Mingyuan.