Definition of nappy bag in US English:

nappy bag

noun

British
  • 1A large bag for carrying diapers and other items used to care for a baby; a diaper bag.

    ‘no nappy bag is complete without a bottle of antibacterial hand gel’
    1. 1.1 A disposable bag for throwing away soiled diapers.
      ‘a supply of free scented nappy bags and wet wipes’