Plastic has almost become contradictory in its usage. In one sense, the term plastic is used to describe something that is fluid, flexible, or easily worked; in short, a positive attribute. In slang, however, due to the cheapness of production and unoriginality of use in design, plastic has a more entrenched meaning of bland, artificial, cheap, worthless.
We used to use this word to describe certain affluent types that had too much money and not enough personality--the kind of people that never were graced with the challenge of developing character (i.e., PlasticMan)