Select the most appropriate meaning of the given idiom.

An arm and a leg

A large amount of money

The most appropriate meaning of the idiom "an arm and a leg" is a large amount of money.

The other options are incorrect.

• "Willing to do something immediately" does not mean a large amount of money. Being willing to do something immediately means that you are eager to do it without delay.
• "At the service" does not mean a large amount of money. Being at someone's service means that you are willing to help them.
• "In a backward and forward motion" does not mean a large amount of money. Moving in a backward and forward motion means that you are moving back and forth.

The idiom "an arm and a leg" comes from the idea of something being so expensive that it is almost impossible to afford. In the same way, something that costs an arm and a leg is something that is very expensive. For example, you might say that a new car costs an arm and a leg.