1. Someone who despite multiple explanations is unable to grasp simple concepts.
2. Common Sense 'Road Kill"
3. Lacking basic and/or needed skill-sets to succeed in current work, much less advanced position, or someone who is employed well above what subordinates or co-workers believe they are intellectually able to handle
4. Darwin Award candidate
You rarely see a dead squirrel on the side of the road. So a slow squirrel is one who probably should have been removed from the gene pool.
E.G. He/She is a slow squirrel.
Anyone who has made honorable mention or received a Darwin Award