From ARRSEpedia

Common mis-spelling of Bergan, the British Army nickname for a rucksack, derived from the manufacturer of the orginal version 'Bergan of Norway'. It never has enough room to fit the kit you want, however always weighs twice as much as the sum total of all the kit you have managed to cram into it. The penis bergen is issued specially to CROW as a place to store their admin Vortex. It resembles a penis as it is stacked to the brim with useless kit, then some more is added and the lid buckled over it. Bergens are frequently left behind on a Bug Out.