In some seriously rural English dialects there's a confusion between positive and negative terms: e.g. 'I doubt' can mean 'I am certain'.
Confusion between positives and negatives is actually very common in most languages and dialects, which is part of why double negatives are also so common- they reinforce the negative, which is easily ignored.
And actually, words change their meaning to the opposite all the time. See the etymology of "like"- it originally meant "to be pleasing to X" (like Spanish gustar) but then evolved into its current meaning. Or consider "very", which comes from a word which meant something similar to literally.
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