The Nanny Dog Myth was invented by the president of the Staffordshire Bull Terrier Club, Lillian Rant, around 1970 (See: The Nanny Dog Myth Revealed). The term refers to the pit bull’s allegedly (and erroneously referenced) unique ability to be overly gentle and patient with children. The term caught fire in the mid 1980s, after the first U.S. city adopted a pit bull law, and has been widely disseminated ever since. A good example of its usage is found on an Oprah community page:

allen76 is right. PITS were called “THE NANNY DOG” from the 1920’s to the end of the 1940’s and were THE MOST POPULAR FAMILY DOG IN THE USA. Gayle and Oprah should not speak about topics they have no education about (like Pit bulls). Pits are WAY DOWN on this years bite/attack list. Labs are at the top. – evie156

There were NO fatal dog attacks by labradors in 2009.