> What does "to coin a phrase" mean?

To coin = to impress plain metal into the shape of a valuable and shiny new coin...

and so by extension to coin a phrase = to impress plain language into a valuable shiny new expression...

(the untrusim residing in the fact that this expression is dragged out like a dead chicken trailed through the dust when someone is about to offer up just a trite and hackneyed old saw)