Describes a feeling that has taken hold of someone without any thinking behind it — a feeling that does not come from facts, and that no argument can reach. The usual partners are strong, dark feelings: unreasoning fear, unreasoning hatred, unreasoning panic, unreasoning anger, unreasoning loyalty. "An unreasoning fear of the dark." The important part is what the word is telling you about the person. It is not that they thought about it and got it wrong; it is that no thinking happened at all. The feeling simply arrived. That makes it different from unreasonable, which is the word you want almost every other time. Unreasonable describes demands, prices, requests, behaviour, people — things you can argue with ("an unreasonable boss," "unreasonable rent"). Unreasoning describes only feelings, and it says those feelings are beyond argument. You would never call your landlord unreasoning. This is a literary word. You will meet it in novels, in newspapers writing seriously about mob violence or prejudice, in a sentence like "an unreasoning hatred of strangers." You will very rarely hear it in a conversation. In speech, people say "he's just scared, there's no reason for it" or "blind panic." Blind is in fact the everyday word doing the same job — blind fear, blind hatred, blind loyalty — and it is worth knowing that pairing.