An Arm and a Leg
It's something that is extremely expensive. For instance, if you are looking to buy a house in the future, the common idiom An Arm and a Leg might be used to describe the cost.
The phrase origin for this saying is unknown. There you go, done. Wait, maybe there's a little information about the origin:

Phrase Origins
What we do know about this expression is that during World War 1 in 1914, soldiers returning home to their families would sometimes be missing a limb that was lost during the war.

It's possible that the saddened family members expressed how the war came at a high price; it cost them an arm and a leg.

This, however, is not able to be verified.

The Expression Used In Sentences:

"Buying a brand new car is going to cost us an arm and a leg! Maybe we should just take the bus to work."

"We're paying an arm and a leg for these excessively high medical bills, we should have gotten health insurance!"

"If our son keeps using the cell phone as much as he has been, we'll be paying with our arms and legs by the end of this month!"
Common Phrase Origins and Meanings > An Arm and a Leg
Learn more common phrases before they run out of them... at the phrase store. Haha! Pretty funny, right?
Note: For a lot of the popular expressions you are searching the origins for, the exact date these phrases originated is very difficult to find. What's shown is the around the earliest date that the saying was being used in a book, poem, or newspaper in.

Obviosuly if it's being used in a newspaper or poem or whatever, it's a common saying among the people already, thus, the origins are older. At least you can get a rough estimate on when it orignated.

And why are you down here reading this? Go back to the top and look for more common expressions! There's a whole lot of 'em. And it won't even cost you any of your limbs to do it
- According to a study found here, your arm span can be used to accurately measure your height!

- The 4 main bones in your leg are the femur (upper leg), patella (knee cap), tibia (lower leg), fibula (lower leg).

- The 3 main bones in your arm are humerus (upper arm), ulna and radius (forearm).

- The funny bone is not actually a bone at all, but a nerve near the ulna bone in our arms.

- You can't walk without legs. Or jump.

Source: Wikipedia - Ulnar Nerve
Meaning of phrase Arm and a Leg.
Origin of the expression Arm and a leg.
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