Thick

"Oh, come on, no teacher's going to fall for that," said Ron. "They'd have to be really thick..."
- Lockhart, anyone? (CS9)

"How thick can you get?"
- Ron, referring to Crabbe and Goyle (CS12).

"How thick would Harry have to be, to go looking for a nutter who wants to kill him?"
- Ron, discussing escaped convict Sirius Black (PA5)

"How thick are you, Wormtail?" (OP28)