Why was the term "coined", coined "coined"?

Answer 1

rikklyn correctly identifies a connection between the words "coin" and coign/quoin. But the expressions "coin a word" or "coin a phrase" have nothing to do with "preparing it for a print."

Instead the expression is based on the simple sense "stamp out of a sheet of metal". It's first known use is by an Elizabethan author (usually thought to be George Puttenham) in 1589, to make fun of the practice of some to try to appear learned by artificially invent new words by taking them from Latin. Here's the quote:

"the common fault of young schollers not halfe well studied . . . [who] will seeme to coigne fine wordes out of the Latin, and to vse new fangled speaches, thereby to shew themselues among the ignorant the better learned.
("The Arte of English Poesie")
http://everything2.com/index.pl?node=to%20coin%20a%20phrase

some more explanation:
http://www.takeourword.com/TOW150/page2.html

In fact, the MODERN use of the expression "to coin a phrase" (1940 or a bit before) as a false apology or joke after using an OLD, hackneyed expression, has a bit of the same spirit.

Answer 2

When you 'coin' something, you create something brand, shiny new. And the shiny, newest thing people could think of a couple of hundred years ago, were new coins. So whatever was bright and new, was refered to as being 'coined', as in 'to coin a phrase'. In that case, the phrase would be something bright, new and clever.

Answer 3

I thought it was quoined. Turns out that's a variation of coined (or the other way 'round) anyway: In Printing: A wedge-shaped block used to lock type in a chase.
So to coin (or quoin) a word or phrase would mean to set it for print.

Answer 4

Usually it is used in the context of "coining an idea", or he"coined the concept", meaning created and absorbed,
that project is his baby.

Hope this helps.

Answer 5

When a new word is "minted", it is just like making money, thus "coined".