Related Clues:

• Cement-headed
• No rocket scientist
• Like a blockhead
• Dimwitted
• Short on sharpness
• Blockheaded
• Like osmium, more than any other known element
• Hard to penetrate
• Crammed
• Needs another way?
• Concentrated
• None too swift
• Hardly suited for Mensa
• Slow to understand
• Not the brightest bulb on the Christmas tree