Related Clues:

• Not the sharpest pencil in the box
• Not very sharp
• A few bricks shy of a load
• Highly concentrated
• Tough to get through to
• Hard to get through to
• Like a thicket
• Like heavy fog
• Closely crowded together
• Knuckleheaded
• Lodged
• Really crammed in
• Unable to continue
• Slow to catch on
• Crowded together