"Inauthentic" is more commonly used than "unauthentic", at least these days, but they are both valid. Several sources (World Wide Words for example) suggest what prefix the negative form of an English adjective takes depends on its roots.

In general, words take un- when they are of English (Germanic) origin and in- if they come from Latin. (The forms im-, il-, and ir- are variations on in-.)

So logically the negative of "authentic" takes "in". "Inauthentic" it is! Problem solved. Or is it? A quick Google Ngram lookup shows that "unauthentic" was much more common before the 1950s and "inauthentic" had barely occurred by that point. Why does "unauthentic" seem to be the older, more traditional form of negation, making "inauthentic" seem inauthentic?

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This popped up on the side bar after I posted my question. For reference:

Is there a reason for the prefix change of in-/un- in about the 60s period for these words?

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    You appear to ask why unauthentic was commonly used till the ‘60s when inauthentic became more popular and still is. Am I correct?
    – user 66974
    Commented Mar 15, 2023 at 19:40
  • Or is this a case where the prefix of a word used to negate its meaning has changed over time, because English uses a variety of prefixes to negate the meaning and we just arbitrarily changed our preferences? Commented Mar 16, 2023 at 18:17

3 Answers 3


'un...' is the common negative for Anglo Saxon and 'in' does the job for Latin derived words. Unfortunately, the word 'authentic' is of neither derivation: it comes from the ancient Greek 'authentikos' ('αυθεντικος'). The standard negative is the so-called 'alpha privative', using the prefix 'a-', as in 'atheist', adding an 'n' to make it 'an', as in 'analgesic'. Strictly following this rule, it should be, therefore, 'anauthentic'.

However, outside the sciences the Greek side of things has slipped away and the application of Latin negative forms has wobbled. So dictionaries like the Cambridge English Dictionary online accept both 'inauthentic' and 'unauthentic' with exactly the same definition. Most people are familiar with use of 'in' or 'un' as negative prefixes. They are unlikely to say or write inwashed rather than unwashed or inlocked rather than unlocked. The CED online allows some latinate words to be negated by both but rejects others: for example, it allows only 'in' as the negative prefix for 'audible'.

Does this matter? No. The English language is highly open to change. The dictionaries pay most attention to usage and have much better access to usage by the mass writing (and speaking) population than a hundred years ago.

So the only rule to follow is "check a good dictionary".

  • 5
    As Etymoline notes, though the word started out in Greek, it got laundered through Latin (and then French) before coming into English.
    – alphabet
    Commented Mar 16, 2023 at 3:01
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    I'm going with "anauthentic" from now on for the same reason I go with "octipodes" even though both are wrong by modern stadards Commented Mar 17, 2023 at 1:12
  • 2
    Me fail English? That's unpossible!
    – CJ Dennis
    Commented Mar 17, 2023 at 3:58
  • @ScottishTapWater: You should certainly feel free to say octopodes if you like, but you might want to read english.stackexchange.com/a/138236/14775 first.
    – ruakh
    Commented Mar 17, 2023 at 7:30

There are many, many common Latin-derived words that take un-: unfortunate, unpopular, unusual, unable, et cetera. I don't think this rule is particularly accurate.

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    Would you go so far as to describe it as being inaccurate? Commented Mar 16, 2023 at 20:57
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    @BenHocking Or undependable.
    – alphabet
    Commented Mar 16, 2023 at 21:06

'Un' is for adjectives based on past tense verbs. 'In' is for modifying related adjectives.

Examples: indirect / undirected; indecisive / undecided; incorrect / uncorrected; inattentive / unattended; inauthentic / unauthenticated; invalid / unvalidated / invalidate(d)*.

Curious though is invalidated, which is the past tense of the verb invalidate and not an adjective.

  • 3
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    – Community Bot
    Commented Mar 16, 2023 at 14:31
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    Unusual. Incapacitated. Uncouth. Unsafe.
    – Sneftel
    Commented Mar 16, 2023 at 17:32
  • 1
    You're not answering the question, though. Please take the tour to familiarize yourself with this platform. Welcome to ELU!
    – Joachim
    Commented Mar 16, 2023 at 17:49
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    @Sneftel Your four examples don't refute the answer. "Incapacitated" is the past participle (or past tense) of "incapacitate", not an opposite of *capacitated. Your other 3 examples have nothing to do with verbs, and so are beyond this answer's scope.
    – Rosie F
    Commented Mar 16, 2023 at 18:02
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    In my mind, "unvalidated" and "invalidated" mean different things. "Unvalidated" means something never has been validated, while "invalidated" means that its validation has been refuted (it was valid and now it is not). Commented Mar 16, 2023 at 19:39

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