Wiktionary
it bag
n. (context colloquial 1990s-2000s English) A kind of expensive designer handbag that is a must-have for fashionable people.
Wikipedia
It Bag
It Bag is a colloquial term from the fashion industry used in the 1990s and 2000s to describe a brand or type of high-priced designer handbag by makers such as Chanel, Hermès or Fendi that becomes a popular best-seller.