sandbag

Concise Oxford English Dictionary © 2008 Oxford University Press:
sandbag/ˈsandbaɡ/

▶noun a bag of sand, used for defensive purposes or as ballast in a boat. ▶verb (sandbags, sandbagging, sandbagged)

• 1 (usu. as adj. sandbagged) barricade with sandbags.

• 2 hit or knock over with or as if with a blow from a sandbag.

• 3 N. Amer. informal bully.

• 4 deliberately underperform in a contest to gain an unfair advantage.

– derivatives
sandbagger noun.