Deeply ignorant

I was impressed by your answers to my "lonesome valley" type question. Thanks.

Here's another one. Have you got any good phrases for someone who holds under-informed opinions tenaciously? Or for the act of holding tenaciously to such opinions?

Spark

'Bullheaded,' 'pigheaded,' 'muleheaded,' 'mulish' and 'willful' are mentioned in the archives.

I love a phrase used by Truman Capote. He said someone was, "The most deeply ignorant man I've ever met."