Stupid.

He’s being thick; he doesn’t understand.

This idiom or expression like phrase means:

Stupid.
Someone whose brain is (implied to be) the size of a pea.

Whose pea-brained idea was this anyway?

Who is the pea-brained moron standing in the middle of the street in rush-hour traffic?

This idiom or expression like phrase means:

Stupid.
Simple.

We need a bone headed solution, nothing complicated.

Also:
A person who is so set in his ways and ideas that nothing gets through his head.

This idiom or expression like phrase means:

Stupid.