To have something to say but find it difficult to express, often due to embarrassment, fear, or a sense of inappropriateness; to be tongue-tied or at a loss for words when one has a lot to explain.
他心里有很多委屈，但当着大家的面，却有口难言。
He had many grievances in his heart, but in front of everyone, he found it difficult to express them.
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