As readers, we recognize prefixes, like dis-, in-, non- and un-, as expressing negation. We immediately know that “unfair” means “not fair.” However, there are some clear exceptions to these rules. Such anomalies can cause confusion for a few reasons. For one, the prefix in- also literally means in, such as inquire, inclose, and insure. The word impromptu for instance comes directly from the Latin phrase in promptu that means “in readiness.”
In other cases, the original root word was lost to time. For example, insipid, meaning bad taste, comes from the word sapid, meaning tasteful. Today insipid is used much more often than sapid.
One particularly tangled example of negative prefixes is the pair ravel and unravel. The word ravel comes from the Dutch word ravelen meaning to tangle or unweave. The word simultaneously means to disentangle and to entangle because in sewing as you unwind a thread, it becomes tangled. Debunk was originally a neologism by author William Woodward in his 1923 book Bunk, whose main character “de-bunked” nonsense or illusions, basically bursting bubbles.
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Are there other words that you come across that were contrary to your expectation?