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The phrase 'costs an arm and leg' is used to describe anything that is considered to be extremely expensive or excessively pricey.

If a person thinks the cost of something is unreasonably high, they might use this common idiom to describe the price.
The origin of this phrase is unclear. On that note, here are a few guesses as to where it may have come from: One theory is that this saying originated from the early 20th century, possibly during one of the major World Wars. The idea being that soldiers, because of their heavy involvement in war and being in the line of fire, can sometimes lose a hand, foot, leg, or arm. Thus, the war could literally cost the person their arm or leg, which is a high price to pay.

Another guess is that this phrase may simply derive from older expressions that also use arms and legs as ways to describe a high cost. For example, there's an expression that goes something like 'I would give my right arm,' that dates back to at least the late 18th century. The earliest recording of this is written in a magazine called The Lady's Magazine: Or Entertaining Companion for the Fair Sex, Appropriated Solely to Their Use and Amusement, 1790, and it reads:

"This is my sole desire—my only passion; and in order to gratify it, I would give my right arm, and my
entire fortune."

Thus, there's a possibility that the phrase 'costs an arm and leg' comes from older, similar expressions like the one mentioned above. Whatever the case, the earliest recording I could find for this particular phrase is around the mid-20th century. An example of that comes from the comic section in the Long Beach Independent newspaper, 1951, where a narrator from one of the comics says:

"It cost them an arm and a leg to fix up a rumpus room for junior!"

Reference: ThePhraseFinder, newspaperarchive contained a digital copy of the newspaper with the expression in the quote above.
* Buying a brand new car is going to cost us an arm and a leg... maybe we should save money by riding a bike or taking the bus to work.
Note: Tracking the idiom origins, or the history of particular expressions, usually results in them being found written in old books, poems, or newspapers. Typically, the origins are older than these references, but regardless, these phrases will still be quoted in order to give you an idea on how long ago a phrase was used.

The history of expressions are often unclear, and in such cases, a few of the popular theories surrounding them will be given. Sometimes, I'll even toss in a plausible guess as to its history or how it may have potentially formed. My intentions here are to give you a better understanding into a phrase's origins, and a comprehendible definition as to what these sayings mean.
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