A Thinking Cap is an item in Mid-World that is worn by many creatures and people on their heads, much like a hat.
Many of the robotic creations of North Central Positronics wear such devices. In fact, the first time the term is mentioned is by Roland in The Waste Lands when referring to Shardik's metallic cap that also has a satellite dish. The Wolves also have thinking caps and is their weakest point. This fact was discovered when Jamie Jaffords and his friends stood against the Wolves.
A different form of a thinking cap is also worn by Walter in The Dark Tower as a means to prevent Mordred from reading his thoughts. It is the same type of Thinking Cap worn by the can-toi in Devar-Toi. This ploy fails as Mordred is smarter than Walter expected, and so Mordred hears what Walter thinks. Walter meets his end after Mordred commands Walter to feed himself to him.