Am I sinful for becoming angry and threatening my sick mother, who wanted to prepare food for me, and for telling her in an angry tone: "If you prepare it, I might not eat it"? And is conveying my father's words about my mother to her considered backbiting (namimah)? What is the meaning of namimah, and what is its expiation?

Informing the mother about what the father criticizes her for is a vile backbiting (namimah (definition)); because it leads to corrupting the relationship between the parents. It combines the major sin of backbiting with the major sin of undutifulness (disobedience) to parents. It is not permissible to address the mother harshly; rather, she must be addressed with gentleness, softness, ادب (proper etiquette), veneration, and humility.