What is "mailbags"

Usage examples of "mailbags".

None of the things that Dalziel was always telling him, such as he should eat more red meat, or that a university degree was what any convict could get between jerking off and sewing mailbags, seemed to apply.

Gil had ridden mail couriers on a space-available basis himself a time or two, and suspected that the lieutenant had probably been sleeping on top of the mailbags for a day or more in order to make it to Galcen tonight.