The Rat Pack was a nickname given to a group of 1950s entertainers, which included Frank Sinatra, Dean Martin, Sammy Davis, Jr, Joey Bishop, and Peter Lawford. The Rat Pack often performed in Las Vegas and were part of the reason that Las Vegas became such a popular entertainment destination.
The Rat Pack made several movies together which include: Ocean's Eleven and Robin and the Seven Hoods.
A pack obviously means a group of some sort, and a rat is a dubious person or scoundrel. The term "rat pack" was assigned to the group by Lauren Bacall, after seeing them return from a night of partying.