Related Clues:

• Like osmium, more than any other known element
• Hard to penetrate
• Inane
• Moronic
• Idiotic
• Crammed
• Needs another way?
• Concentrated
• None too swift
• Hardly suited for Mensa
• Slow to understand
• Not the brightest bulb on the Christmas tree
• Sold-out show
• Thickly populated
• Bubbleheaded