To be speechless or dumbfounded — so surprised or astonished that one is left with an open mouth and unable to close it; equivalent to being stunned.
彼のとんでもない言い訳を聞いて、開いた口がふさがらなかった。
I was left speechless hearing his outrageous excuse.
その値段を見て、開いた口がふさがらないほど驚いた。
I was so shocked by the price that I was speechless.
不正が発覚したとき、彼女は開いた口がふさがらない様子だった。
When the fraud was uncovered, she looked utterly speechless.
冗談のつもりで言った言葉が本当だったと知り、開いた口がふさがらなかった。
I found out that what I had said as a joke was true, and I was left speechless.