An idiom meaning to be astonished or deeply impressed to the point of being speechless; to marvel at something.
彼女のヴァイオリン演奏には本当に舌を巻いた。
I was really astonished by her violin performance.
この小さなレストランの料理の味には舌を巻くほどだ。
The food at this small restaurant is astonishingly good.
専門家でさえ彼の分析の鋭さには舌を巻いていた。
Even the experts were astonished by the sharpness of his analysis.