expression, godan verb (-ru)
be at a loss for a reply; be stuck for an answer
Used when someone cannot think of an appropriate response, often due to embarrassment, surprise, or a difficult question. Similar to 返答に窮する.
See also: 返答に窮する
突然の質問に、彼は返答に詰まった。
He was at a loss for a reply to the sudden question.
Synonymous; 返答に詰まる is slightly more common in everyday speech, while 返答に窮する can sound a bit more formal or literary.
Compound of 返答 (reply) + に + 詰まる (to be blocked, to be stuck). The verb 詰まる here figuratively means being unable to proceed with words.