There are so many weird idioms and expressions.
We will often know what they mean – like this one. If somebody says that their "car cost an arm and a leg" we know it has been expensive.
We completely understand that the person is not limping around on one leg with the support of only one crutch.
In short: Something that is very expensive.
Like most old expression, we do not know exactly where it came from.
There are, however, a lot of possible origin stories.
One unlikely reason for why this expression came to be goes back to George Washington’s time. Having your portrait painted became very popular. And painters were often charging more for legs and arms because they were more difficult to get into the right propotions. (Just look at the first AI images.)
Painters would therefore charge less if one arm or leg was hidden away from the painting. Like they were behind a chair or hidden under a cape.
There is some truth to this.
Painters did in fact charge more for full body portraits as opposed to just the face. There is, however, no evidience that shows that the expression originates from this.
A more convincing explanations about the origin of the expression "cost and arm and a leg" derives from the Irish halfpenny coins in the 17th century.
In the year 1680, King Charles the 2nd had Sir Thomas Armstrong and Colonel George Legge manufacture halfpennies to use in Ireland. These coins were later used by Irish emigrates in America.
The names of the 2 manufacturers were Mr. Armstrong and Colonel Legge, Arm and Leg.
Hence the expression "it will cost you an arm and leg" referring to an Irish halfpenny.
The last plausible theory is the simplest one.
An arm and leg is considered very precious and therefore there is a very high price to losing it.
During the World War II, many servicemen lost limbs and therefore said the price of war was "an arm and a leg."
Other countries have similar expressions related to cost.
In France they say that "Ça coûte les yeux de la tête" which translates directly to "it cost the eyes of the head."
In Danish, they say that "det koster det hvide ud af øjnene" which means "it costs the white out of the eyes."
Yes, eyes are valuable, too.
