To coin a phrase

I have been laboring under the increasingly heavy burden that I didn't really know what this phrase means. Does it mean 'to use a phrase everyone knows' or 'to make a new phrase'? Google time.

Well, are you descriptive or prescriptive, because people use it both ways, contributing to my confusion. Indeed, you can make an argument for both usages from the words themselves. Here's OUP on different usages.

Wordorigins.org (scroll down) says it's 'make a new phrase'.

Similar argument on Take our word for it (scroll down).

That seems to be the consensus, which brings me to say 'Why bother using it?'. How often do I 'coin a phrase' as opposed to 'use a hackneyed, over-exercised phrase'? :-)

But there you have it.