expression, suru verb (special)
be at a loss for a reply; be stuck for an answer
Used when someone cannot think of a suitable response, often due to embarrassment or being caught off guard. Similar to 返答に詰まる.
See also: 返答に詰まる
突然の質問に、彼は返答に窮した。
He was at a loss for a reply to the sudden question.
The demand was so unreasonable that I was stuck for an answer.