What does the lyrics called mistress mean?

The lyrics of "Call Second Grandma" were sung in the movie (San Xiao), and Tang Bohu sang it to Second Grandma in the play. The song "Call Second Grandma" was sung by Tang Bohu and his second sister. In pursuit of Chou-heung, Tang Bohu entered Washington disguised as a slave. His second sister has married in Washington and recognized Tang Bohu. On the premise of not telling the truth, Second Grandma accused Tang Bohu's generation of gifted scholars of being slaves, and Tang Bohu asked Second Grandma to help hide it.