Walls have ears; meaning, one must be careful about what is said, as there may be unseen or hidden listeners (eavesdroppers).
你们在房间里讨论重要事情时，一定要小心，属垣有耳，以免被外人偷听。
When you're discussing important matters in the room, you must be careful, walls have ears, so as not to be overheard by outsiders.
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