"On 18 you've got to drive it up a gnat's ass"

This quote by Greg Norman is referring to the problem of a golf shot. The phrase "drive it up a gnat's ass" is a metaphor for a shot that is extremely challenging to make. It suggests that the shot is so accurate that it would be almost impossible to strike it in the precise area that is needed. The "18" in the quote is most likely describing the 18th hole of a golf course, which is normally the most challenging hole. This quote is an amusing method of revealing the problem of a golf shot, and it is a suggestion that golf is a game of precision and precision. It is a pointer that even the most skilled golfers can discover themselves in difficult situations, and that it takes a lot of skill and practice to be able to make the most hard shots.