Argumentum ad lapidem (Latin: "appeal to the stone") is a logical fallacy that consists in dismissing a statement as absurd without giving proof of its absurdity.
Ad lapidem statements are fallacious because they fail to address the merits of the claim in dispute. The same applies to proof by assertion, where an unproved or disproved claim is asserted as true on no ground other than that of its truth having been asserted.
The name of this fallacy is derived from a famous incident in which Dr. Samuel Johnson claimed to disprove Bishop Berkeley's immaterialist philosophy (that there are no material objects, only minds and ideas in those minds) by kicking a large stone and asserting, "I refute it thus." This action, which is said to fail to prove the existence of the stone outside the ideas formed by perception, is said to fail to contradict Berkeley's argument, and has been seen as merely dismissing it.
Example
Speaker B gives no evidence or reasoning, and when pressed, claims that Speaker A's statement is inherently absurd, thus applying the fallacy.