AntiAutonyms
The Same, Only Different

An antiautonym is one of a pair of words which mean the same thing, although one word seems, for some reason, as though it should be the "opposite" of the other. This concept is related to, but different than, an autoantonym, which is a single word which means both one thing and its opposite.

Some examples of antiautonyms may simply be considered incorrect speech, but they are collected here nonetheless. Examples include:

• BONED / DEBONED: having had the bones removed.
• BOUCH / DEBOUCH: to open out into a wider area.
• CATEGORICAL / UNCATEGORICAL: Both words are used to mean "sweeping", "broad" or "without exception". "CATEGORICAL" is correct, while "UNCATEGORICAL" is colloquial.
• COVER / UNCOVER: A newspaper can both "uncover" a scandal and "cover" a scandal. In both cases, the news about the scandal is reported.
• EBRIATE / INEBRIATE: Both words mean "intoxicated" (or should I say "toxicated"?). The "in-" prefix actually is not used in its negating sense, but to mean "into".
• EXHAUSTIVE/ INEXHAUSTIBLE: Both words may be used to describe someone's knowledge or expertise, and yet they seem to be negating each other. Here, at least, we have an explanation, since exhaustive is meant to suggest that the person has exhausted the subject (and hence, learned all there is to know), while inexhaustible is meant to suggest that you could never come to the end of that person's knowledge of the subject. So while these are actually different ideas, strictly speaking, in most people's minds they are used in the same context.
• FLAMMABLE / INFLAMMABLE: Both words mean "liable to burn". "INFLAMMABLE" is the original form, and "FLAMMABLE" a modern invention for the sake of simple folk.
• HERITABLE / INHERITABLE: Again, the particle "in" does not indicate negation.
• I COULD CARE LESS / I COULDN'T CARE LESS: The first phrase is colloquial, and probably the result of mishearing the correct second phrase.
• LOOSEN / UNLOOSEN: both mean to make loose.
• PRICEY / PRICELESS
• REGARDLESS / IRREGARDLESS: The second word is a colloquialism that is extremely irritating to linquistic curmudgeons.
• SHAMELESS / SHAMEFUL: you'd almost swear these words are opposites, (you can even convince yourself that "shameless" can be used as its own opposite). However, these words are most commonly used to describe a shameful deed done by a shameless person. The shame that the deed would engender in an ordinary person does not affect the shameless person, who by some peculiarity of spiritual genetics cannot experience this emotion.
• SHOULD / SHOULDN'T: in rhetorical questions:

  Should we crush these signs of rebellion? -- Yes!.

  Shouldn't we crush these signs of rebellion? -- Yes!.

• VALUABLE / INVALUABLE: the second word actually suggests something of greater value.

It's not really impossible for something to be the same as its "opposite". It's unusual for words, but here's a silly example of an antiautonymical sentence:

He read half of the words on the sign.

He didn't read half of the words on the sign.

It is not the case that he read half of the words on the sign.