KnowYourPhrase.com - Common Phrases and Idioms - Meanings and Origins
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 origins of this phrase are unclear. There are, however, a few theories 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 possibly lose a hand, foot, leg, or arm. Thus, the war would literally cost the person their arm or leg, which is a high price to pay.

Another theory is that this phrase may simply derive from older expressions that also use the terms 'arms' and 'legs' as ways to describe a high cost. For example, there's an expression that goes 'I would give my right arm' that dates back to at least the late 18th century. An early example 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, perhaps the phrase 'costs an arm and leg' derives from older, similar expressions like the one mentioned above. Whatever the case, the earliest citation I could find for this particular phrase being used in writing is around the mid-20th century. For example, in the comic section of a Long Beach Independent newspaper, 1951, a narrator from one of the comics says:

    "It cost them an arm and a leg to fix up a rumpus room for junior!"
* Buying a brand new car is so expensive, it is going to cost us an arm and a leg! Maybe we should ride bikes or take the bus to work in order to save money.
Note: There are times when a phrase's origins are uncertain, like the one mentioned above. In cases like this, I'll list the popular and/or plausible theories that talk about how a phrase may have originated. If no theories are listed, then I'll try to at least include the oldest known quote of an expression being used in writing. These quotes can give you an idea on at least how far back in history an expression goes.

More and more phrases will be added to this site, so check back another time if you can't find one that you're looking for.

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