A big bag knife, comprising - a back part and a front part, each comprising a back end (21, 31) and a front end (22, 32) and each having a certain length, wherein the back part extends around at least part of the length of the front part, - a blade provided at the front end of the front part, - a sliding mechanism that attaches the front part to the back part in a sliding way, and that enables the front part to be slid in view of the back part between a retracted position of the knife in which the blade is sheathed by the back part and is inaccessible to the user of the knife, and an extended position of the knife in which the blade is situated outside the back part and enables the user to cut the big bag, wherein a front handgrip is mounted on the front part in the vicinity of the back end thereof, with the front handgrip adapted to be gripped by a hand of a user of the big bag knife and to slide the front part in view of the back part between the retracted and the extended position of the big bag knife; wherein a back handgrip is mounted on the back part in the vicinity of the back end thereof, this back handgrip being adapted to be gripped by means of the other hand of the user of the big bag knife; characterised in that the back part extends around the complete length of the front part in the retracted position of the big bag knife, and the front part is able to slide in and out of the back part between the retracted and the extended position of the big bag knife. - a back part and a front part, each comprising a back end (21, 31) and a front end (22, 32) and each having a certain length, wherein the back part extends around at least part of the length of the front part, - a blade provided at the front end of the front part, - a sliding mechanism that attaches the front part to the back part in a sliding way, and that enables the front part to be slid in view of the back part between a retracted position of the knife in which the blade is sheathed by the back part and is inaccessible to the user of the knife, and an extended position of the knife in which the blade is situated outside the back part and enables the user to cut the big bag, wherein a front handgrip is mounted on the front part in the vicinity of the back end thereof, with the front handgrip adapted to be gripped by a hand of a user of the big bag knife and to slide the front part in view of the back part between the retracted and the extended position of the big bag knife; wherein a back handgrip is mounted on the back part in the vicinity of the back end thereof, this back handgrip being adapted to be gripped by means of the other hand of the user of the big bag knife; characterised in that the back part extends around the complete length of the front part in the retracted position of the big bag knife, and the front part is able to slide in and out of the back part between the retracted and the extended position of the big bag knife. wherein a front handgrip is mounted on the front part in the vicinity of the back end thereof, with the front handgrip adapted to be gripped by a hand of a user of the big bag knife and to slide the front part in view of the back part between the retracted and the extended position of the big bag knife; wherein a back handgrip is mounted on the back part in the vicinity of the back end thereof, this back handgrip being adapted to be gripped by means of the other hand of the user of the big bag knife; characterised in that the back part extends around the complete length of the front part in the retracted position of the big bag knife, and the front part is able to slide in and out of the back part between the retracted and the extended position of the big bag knife.