The word tank has several meanings:

1. A large container for holding liquids or gases. A fish tank holds water and fish. Cars have gas tanks that store fuel. Water towers are giant tanks that provide water pressure to neighborhoods. When something tanks up, it fills with liquid, like when you fill a car's gas tank before a long trip.
2. A heavy armored military vehicle that moves on metal tracks instead of wheels and fires powerful guns. Tanks were invented during World War I to cross rough battlefield terrain and break through enemy lines. Their thick armor protects soldiers inside from gunfire. Modern tanks remain important in ground warfare, combining mobility, firepower, and protection.
3. To fail completely or perform very poorly. When a movie tanks at the box office, hardly anyone buys tickets to see it. A student's grade might tank if they stop studying. A business can tank when it runs out of money. The phrase in the tank means something has collapsed or failed: “After missing several games, the team's season is in the tank.”