不能说的秘密

This can be literally translated into English as 'A Secret That Cannot Be Told'. The phrase comes from a song title in Chinese which is very famous among the young, suggesting matters so confidential and personal, too private or sensitive, possibly because if exposed, it might cause distress or misunderstanding. As the saying goes in China “Silence keeps secrets.” It implies a mysterious aura while highlighting personal boundaries that shouldn't be crossed.