小三永久登不上大雅之堂
[xiăo sān yŏng jiŭ dēng bù shàng dà yă zhī táng]
'A Mistress Will Never Be Able to Appear in Legitimate Society.’ It means a woman who has an affair with a married man will never receive social acceptance or legitimize her status in the eyes of society. Note: The term has a negative connotation and isn't suitable for formal discourse.