Generally, the negative form of 'reasonable' is 'unreasonable' with the negative prefix 'un', but I do have come across 'irreasonable' used as the opposite of 'reasonable'. So my question is whether both are acceptable by native speakers of English in communication?

  • 1
    Depending on your intended meaning either "unreasonable" or "irrational" would be better.
    – Hot Licks
    Oct 10, 2020 at 2:38

1 Answer 1


The term "irreasonable" is not acceptable to well-educated native English speakers.

As evidence:

  1. it's not in the Oxford English Dictionary - which is the ultimate arbiter of "good English"; and
  2. automated spell checks (for instance, in Microsoft Word and even in this text box on StackExchange) mark the spelling as incorrect.

The term "irreasonable" is clearly capable of being understood, and a quick Google search shows the term is occasionally used.

However, any person using the term would convey the impression that he or she is either a non-native speaker or a poorly-educated native speaker.

  • 1
    Many thanks TechnoCat. Your answer is quite clear. It's good to me and other readers potentially with the same confusion.
    – Eglantine
    Mar 12, 2021 at 11:01
  • The OED is certainly not the "arbiter of good English". It is a descriptive, historical dictionary which aims to record usage, not to prescribe it. For example, it includes words like "irregardless" and "conniption", but it does have usage labels for them, namely "in nonstandard or humorous use" and "U.S. vulgar". But even that isn't saying that the OED disapproves of the words -- it it trying to record how they are used. Aug 29, 2021 at 16:14

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