A hammer is a tool that consists of a heavy piece of metal at the end of a handle. It is used, for example, to hit nails into a piece of wood or a wall, or to break things into pieces.

If you hammer an object such as a nail, you hit it with a hammer.

If you hammer on a surface, you hit it several times in order to make a noise, or to emphasize something you are saying when you are angry.

If you hammer something such as an idea into people or you hammer at it, you keep repeating it forcefully so that it will have an effect on people.

If you say that someone hammers another person, you mean that they attack, criticize, or punish the other person severely.

If you say that businesses are being hammered, you mean that they are being unfairly harmed, for example by a change in taxes or by bad economic conditions.

In sports, if you say that one player or team hammered another, you mean that the first player or team defeated the second completely and easily.

If someone's heart is hammering, it is beating very fast, usually because they are frightened.

In machines and instruments, a hammer is a part that hits another part. For example, in a gun the hammer causes the explosion which makes the bullet shoot out of it, and in a piano the hammers hit the strings and cause the sounds.

In athletics, a hammer is a heavy weight on a piece of wire, which the athlete throws as far as possible.

If you say that someone was going at something hammer and tongs, you mean that they were doing it with great enthusiasm or energy.

If you say that something goes, comes, or is under the hammer, you mean that it is going to be sold at an auction.