Not the sharpest knife in the drawer
Definition 1 of 1
Pro Tip 1/3Metaphorical Usage
The phrase is a metaphor that implies a lack of mental sharpness, comparing a person's intellect to a dull knife. 
You may need to explain things twice to Henry; he's not the sharpest knife in the drawer.
Pro Tip 2/3Avoid Insults
It can be offensive or humorous depending on tone and relationship. Use with caution to avoid hurting feelings. 
Let's assign the simpler task to Karen; she's not the sharpest knife in the drawer.
Pro Tip 3/3Informal Context
Suitable for casual conversations, but it's not appropriate in formal or professional settings. 
During lunch, Mike joked that he's not the sharpest knife in the drawer when it comes to technology.