Appeal to the stone

Appeal to the stone is an informal fallacy that consists in dismissing a statement as absurd without giving proof of its absurdity.

Speaker A: Infectious diseases are caused by microbes.
Speaker B: What a ridiculous idea!
Speaker A: How so?
Speaker B: It's obviously ridiculous.

Speaker B gives no evidence or reasoning, and when pressed, claims that Speaker A's statement is inherently absurd, thus applying the fallacy.