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a traditional custom or idea adhered to although shown to be unreasonable
a person who obstinately adheres to old customs or ideas in spite of evidence that they are wrong or unreasonable

In an old story, it's said that an ignorant priest, knowing the sound of the Latin Mass but not speaking the language, said the meaningless 'mumpsimus' instead of 'sumpsimus'. When corrected, he is said to have replied, "I will not change my old mumpsimus for your new sumpsimus."
source
I very recently discovered someone I know believes in the flat earth hypothesis.
Have you ever been shocked to find someone you know (and respect) believes in an idea you'd have thought we'd all left behind?
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