The phrase "heavy sandbag" means to have a burden or a problem that is difficult to handle or overcome. It can refer to a challenging situation or a person who causes trouble or puts pressure on others. Full definition
Artillerymen will have to be able to manhandle heavy ammo boxes and MSTs for infantry soldiers will include tasks such as dragging heavy sandbags to simulate helping a fallen comrade and crossing obstacles to simulate working in urban environments. (fitnessandpower.com)
The heavy sandbag should have a place in every MMA workout. (fitnessandpower.com)