Definition of duff bag in US English:

duff bag

noun

slang, historical
  • A heavy-duty cloth bag for boiling puddings, having a looped cord around its neck used for closure and to remove the cooked pudding from boiling water; (hence) something resembling this bag or its means of closure, especially a handle formed by tying the ends of a neckerchief to the tapes of a jumper, used as a means of rescuing a sailor from water.

Origin

Early 19th century.