Not the sharpest knife in the drawer

exp.

Heard in Sligo

Not very intelligent. Somewhat vague or slow-witted.

In use

A: "Do rabbits lay eggs?" B: "Yer not the sharpest knife in the drawer, are yeh luv?!"

Posh English

A: Do rabbits lay eggs? B: You are not the sharpest knife in the drawer, are you, my dear?!