A more palatable insult for...

Do not call people thick skulled or airheaded. if they truly are one egg short of a full basket you can simply call them "A splendid autonomous being that enjoys a resiliency from data ingress into or data contamination of their granite cavernously bereft cranium."

To a casual listener or some one with a limited vocabulary this phrase sounds like a praise or compliment, however upon closer examination it is clear that it is not.

You are in reality saying, in a more elegant fashion, that they are stone headed, empty minded, and incapable of learning.