To have something to say but find it hard to speak; to be unable to voice one's thoughts or feelings, often due to shyness, embarrassment, or fear.
小明想向老师解释迟到的原因，但因为太紧张，一时有口难开。
Xiao Ming wanted to explain to the teacher why he was late, but due to nervousness, he found it difficult to speak for a moment.
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