Yes I agree with Mike, sometimes an unintelligent person is called as thick as a brick/2planks.

Depends on the context. It could mean something thats large in size or build. It can also be used to mean someone who is abit stupid.

a. Relatively great in extent from one surface to the opposite, usually in the smallest solid dimension; not thin: a thick board. b. Measuring a specified number of units in this dimension: two inches thick. 2. Heavy in form, build, or stature; thickset: a thick neck. 3. Having component parts in a close, crowded state or arrangement; dense: a thick forest. 4. Having or suggesting a heavy or viscous consistency: thick tomato sauce. 5. Having a great number; abounding: a room thick with flies. 6. Impenetrable by the eyes: a thick fog.