Flat-Earther Meaning

(idiomatic, by extension) A person who believes or advocates an outlandish, discredited theory; a person who refuses to acknowledge the truth despite overwhelming evidence.

Example: A person who believes or advocates the theory that the planet Earth is flat.
2008, Bob Sweere, "Voice of the Day: Global warming? Yes," News-Leader.com (Missouri), 16 Jun. (retrieved 1 July 2008),
  Global warming is settled, unless you are a flat earther or an energy company apologist.