When some one is constantly reminded of a medial task or process and always forgets it. Then becomes belligerent and defensive and accuses the person who is reminding them of hiding or withholding the information from them, hence sounding retarded.
In the workplace you usually find them in management positions and is a natural progression to micromanagement
Person: Hey Boss, why don't you use your computer to track the shipments?
Forgetard: I am in charge around here and you have never shown me how to do THAT!?!
Person: Sure I have just hit F4 like I showed you Monday.
Forgetard: You have never shown me that, things are going to change around here! All further shipments, sales figures and customer interaction must go through and be approved by me. You are not to be trusted!