expression, suru verb (special)
be at a loss for words; be stuck for words
Used when someone cannot find the right thing to say, often due to shock, embarrassment, or difficulty. Similar to 言葉に詰まる, but 窮する emphasizes being stuck or in a difficult position.
See also: 言葉に詰まる
突然の質問に、彼は言葉に窮した。
He was at a loss for words at the sudden question.
あまりの光景に言葉に窮してしまった。
I was so stunned by the sight that I couldn't find the words.