illogical
irrational / unsound / senseless / illogical
Adjective
This term describes a failure of reasoning where the conclusion does not follow from the premises. It carries a strong connotation of absurdity or intellectual error, often used to dismiss an argument as flawed or nonsensical. In formal contexts, it refers specifically to a violation of the laws of logic or a structural failure in a syllogism. In casual conversation, it is frequently used to describe behavior or decisions that seem irrational or contrary to common sense.