This is a term used in the place of the superstitious phrase "knock on wood" when one is traveling in a motor vehicle and no wood is around. One instead "knocks on car plastic" to affirm the superstition.
Adam - "What's wrong with your teeth?"
Joe - "I can't brush them because I can't find any tooth brushes or tooth paste suitable for me :("
Adam - "Why not?"
Joe - "I'm a raging plasticarian"