Thick means you're not thin, nor fat, but you got some yummy meat on you. My friend for example had a flat belly, but she was thick---she had this padded thighs and arms, but is not that she was fat. J-lo could be a good example---she is not fat, and she is not thin, she is thick. Paula Abdul in her younger days was also thick--she was not fat, but kind of muscular, and thus thick. That girl on vh1, New York---she is not thin, she is not fat, but she is deliciously, thick. Many guys love thick girls.
First of all, I have never once heard a guy describe a girl as "thick" and mean it in a bad way.
Generally, "thick" means big in all the right places. Those places being chest, butt, hips, and thighs. Basically, everywhere but the waist. Even the waist can have some extra meat there too as long as she maintains an overall hourglass figure. It looks best when you can't quite tell whether its fat or muscle or a mixture of both. I would say more than half the guys out there prefer a girl with a body type like this over a skinny girl. So if a guy calls you "thick", that's what he means and he ALWAYS means it as a compliment.