Dar·by and Joan /ˌdɑːbi ən ˈdʒəʊn $ ˌdɑːrbi ən ˈdʒoʊn/ BrE AmE noun British English
[ Date: 1700-1800 ; Origin: The names of a happy long-married couple in an 18th-century poem ]
like Darby and Joan used humorously when talking about an old husband and wife who live very happily together