old Adam
/ˌəʊl(d) ˈadəm/noun
Chiefly with the. The biblical Adam as a representative of the sinful and unregenerate character of human nature, which in traditional Western Christian theology is overcome by a person's acceptance of Christ; ( allusively ) sinful character or nature, especially as the cause of wild and reprehensible behaviour.
Origin
Late Middle English. From old + the name of Adam, after post-classical Latin vetus Adam.