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3.
A large amount of money; very expensive or costly.

Some things are so expensive that they are painful to buy, and cost everything you have.
My new Mercedes cost me an arm and a leg!
by VAKI5 May 10, 2005
 
1.
Figuratively, what one would have to sell or otherwise place as collateral in order to afford something of great expense.

Comes from the fact that, when buying things of great expense, it actually feels as if you are having to offer said appendages as collateral for said item you are purchasing.
Man, I just had to spend an arm and a leg to get my car repaired.
by ke6isf January 31, 2005
 
2.
an outrageously high price, en excessive price
Don't pay an arm and a leg for your internet service, switch to -* insert company name here *- today.
by The Return Of Light Joker October 09, 2007
 
4.
A metaphor for the concept of equivalent exchange meaning 'at great personal cost', in reference to something gained.

The phrase may have originated from the legend of Edward Elric, an alchemist who attempted to revive his deceased mother and later restored his brother's soul at the physical cost of his left leg and right arm, respectively.
'Hmm, I might go and get a tattoo of one of those Ouroboros things.'
'Be careful, it'll cost you an arm and a leg!'
by josephparkins June 02, 2013