决不 is a strong negative adverb meaning 'by no means' or 'under no circumstances', used to emphasize that something absolutely will not happen or is definitively not the case. It carries a tone of firm resolve (like in誓言 or refusal) and is often seen in formal or serious contexts such as promises, denials, and explicit rejections. A near synonym is 绝对不, but 决不 is slightly more literary and emphatic.