To Cost and Arm and a Leg means that something is expensive - very
expensive, not just expensive though, but excessively so.
Interestingly this commonly used phrase has several possible origins
which I will attempt to detail here.
The first possible origin I came across was another slang phrase -
"If it costs a leg!". This was apparently a phrase used by desperados
who were after revenge - even at the cost of a leg.
The second source I came across was the phrase "to chance ones arm".
This phrase is of an army origin and means to do something at great risk
in hope of reward or profit. In this case the risk would be the loss of
rank which - in the case of a non commissioned officer - would be
worn upon the sleeve. So you would be risking losing a stripe from your
arm. Thus, to chance ones arm.
The third, and last, source that I have on this is from the 1800's.
When people commissioned portraits it would have been a lot cheaper to
have a painting done of just your head and shoulders. To get your whole
body painted would have been many times more expensive. Thus to cost an
arm and a leg.
Out of these three possible origins I have to admit that I would be
veering towards the last one as the most likely source for the phrase. But
I do believe that the first two had a measure of influence upon it. I
think that, if the first two are at all true, they probably help to cement
this phrase in to common English use.
Of course this node would not be complete with out the near
obligatory joke, so here goes:
Adam was walking around the Garden of Eden feeling very lonely. So God
asked him, "What is the matter?"
Adam said he didn't have any one to talk to.
God said that he was going to make Adam a companion and that it would
be a woman. He told him, "This person will gather food for you, cook for
you, and when you discover clothing, she'll wash it for you. She will
always agree with every decision you make. She will bear your children and
never ask you to get up in the middle of the night to take care of them.
She will not nag you, and will always be the first to admit she was wrong
when you've had a disagreement. She will never have a headache, and will
freely give you love and passion whenever you want it."
Adam asked God, "What will a woman like this cost?"
God replied, "A woman that special will cost an arm and a leg."
Then Adam asked, "What can I get for a rib"?
The rest is history.
Sources: Brewer's, American Heritage Dictionary, friends.