A "lackwit" in Westeros refers to a person who is lacking intelligence, usually what the real world would call intellectually disabled. They are also often called "halfwits", "simpletons", "idiots", "simple-minded", or sometimes just "simple". Lackwits are disparaged by society.
There are lackwits in all strata of society, from smallfolk to highborn. Despite being lackwit, some highborn can still inherit; for example, Lollys Stokeworth, who becomes the Lady of Stokeworth after the deaths of her mother Lady Tanda and her sister Falyse. Men have been known to marry lackwit heiresses. Male lackwits sometimes become fools.
Uncertainty about Moon Boy
Moon Boy is described as a simpleton. However, several characters wonder whether or not Moon Boy truly lacks intelligence. Moon Boy is known to mock people with deft cruelty. On one occasion, a drunken Dontos Hollard accuses Moon Boy of being a spy of Varys, and that his simple-mindedness is just an act.