Someone who is complacent has become overly content — the junk-food-eating couch potato might be feeling complacent about his health.
The literal meaning of this word's Latin root is "very pleased," but even though complacent people may seem pleased with themselves, we are rarely pleased with them. They are unconcerned by things that should concern them, and they may neglect their duties. A complacent person might be heard saying, "Ehh, don't worry about it!" — when there really is something to worry about.