also: サンドバック
noun
punching bag; punchbag
A hanging or free-standing bag used for boxing or martial arts training. Despite the English source 'sandbag', in Japanese it refers to a punching bag, not a bag filled with sand for flood control.
from English sandbag
ジムでサンドバッグを叩いてストレスを発散した。
I hit the punching bag at the gym to relieve stress.
新しいサンドバッグを買って、家で
I bought a new punching bag and practice at home.