So clear, obvious, or distinctive that it cannot be confused with anything else or doubted.
From the prefix 'un-' (not) plus 'mistakable' (able to be mistaken). 'Mistake' comes from Old Norse 'mistaka,' meaning 'to miscarry' or 'go wrong.'
The prefix 'un-' is one of English's most productive prefixes—you can add it to almost any adjective to create its opposite (unclear, unkind, unhappy), which is why English speakers can understand new 'un-' words they've never heard before.
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