Lackwit

A lackwit is whose mental development has been impaired. Reasons range from birth defects to injuries sustained during childhood. A more polite term to refer to them is simple.

Lowborn lackwits often find themselves in menial positions of servitude. Meanwhile, lackwits that belong to noble houses remain under the care of their families and may be employed in marriage alliances - considered poor matches, obviously - but remain the subject of mockery and shame.

Known lackwits

 * Hodor, a manservant of House Stark, now at the Cave of the three-eyed raven with Bran Stark.
 * Lollys Stokeworth, of House Stokeworth, betrothed to Ser Bronn of the Blackwater
 * {Orson Lannister}, a cousin of Cersei, Jaime and Tyrion Lannister, killed by a mule.

In the books
In the A Song of Ice and Fire novels, some members of House Targaryen are born lackwits due to the generations of inbreeding among its members. The daughter of Prince Daeron Targaryen was feeble-minded and thus passed over to inherit the Iron Throne. A grandson of Walder Frey, Aegon Frey, is employed as a fool, earning his nickname of Jinglebell.