"Possession is nine tenths of the law."
This phrase means that a huge proportion of our laws and legal proceedings are concerned with who owns what. For some reason, I typically hear this phrase used to mean something like "Physically possessing a thing trumps all other claims of ownership", e.g. a family member or friend takes your iPod, you ask to get it back, and he/she chortles, "Possession is nine tenths of the law" while playfully (at least you'd hope playfully) refusing to immediately return the item.
"To coin an expression..."
Coining an expression means inventing that expression, not quoting it! Perhaps somewhere along the line someone has used "to coin an expression" in a sarcastic or ironic sense, knowing they were clearly quoting an existing phrase, but joking as if they were freshly authoring the well-known adage. My impression, however, is that most people who say "to coin an expression" or "to coin a phrase" when they're really quoting, not coining, haven't a clue about the difference between coining and quoting.