The bag (10) comprises a flexible outer (12) having an opening (12A) the edge of which is fixed to the two opposing walls (20A, 20B) of a reinforcing sleeve (18) projecting out from the outer, the bag being able to adopt a flattened configuration in which the sleeve is folded along a main fold line (17) connecting the walls thereof so that said walls lie one against the other, and a configuration of use in which said walls form a canal that keeps the opening open. The main fold line (17) has an end portion (17A) opposite to the opening which is laterally further toward the outside so that in the configuration of use, the sleeve offers a spout delimited by said end portion.
