As a result of a discussion with @Hot Licks on another post, it is apparent that his (American) understanding of the nuances associated with clueless is slightly different to my (British) understanding.
Reference to both Merriam Webster and Oxford Dictionaries would seem to confirm this difference.
At issue is whether clueless is a synonym of oblivious. My inclination, which appears supported by the ODO is that oblivious is about forgetting or being unaware of something, whilst clueless is essentially about knowledge and ability.
Clueless is separately defined by:
ODO: adj having no knowledge, understanding or ability.
But MW has a longer definition:
completely or hopelessly bewildered, unaware, ignorant, or foolish
Referring to what some consider the fount of all knowledge, the OED, which addresses the entire corpus of English as used around the world, there is no mention, under clueless, of unawareness.
To me, to be oblivious means simply to be unaware. Even the greatest scholar could be oblivious of the fact that something was about to happen. But clueless to me suggests a lack of capacity, or knowledge, and it would be quite in error to say that Einstein was clueless even if it was that he did not realise he was about to be attacked by someone lurking in the shadows.
In Britain it would be an insult to call someone clueless, but not necessarily so with oblivious. Is that not the case in America?
Best Answer
I agree that the term is not nearly as grievous an insult in the US as it apparently is in the UK, and can be used in situations where oblivious might also be used.
As the question is about some fairly subtle particulars of usage, and existing dictionary definitions aren't helpful in distinguishing between these usages, I will take the liberty of constructing a working definition to help structure the rest of my answer:
In Sense I, cluelessness about a subject may result from either a (reparable) lack of experience or from an inability to comprehend. However, even in the more severe, lack-of-capacity case the person does not necessarily lack capacity in other areas of life and learning. Examples of this kind of usage:
Although this usage is not necessarily horribly insulting, neither is it quite the same as oblivious. However, the second sense comes much closer to being synonymous with oblivious.
In sense II, clueless can mean that the person lacks knowledge or awareness of a specific circumstance, which can be due to factors entirely beyond the individual's control. I would say that this at least comes very close to synonymy with oblivious. For example:
The following quote illustrates rather well some of the subtleties of the term's usage:
The relative "insulting-ness" of the term might be judged to some extent by how often it is applied in the first person. Looking only at the frequency of use within the American English corpus (here) and British English corpus (here) on Google Ngrams, we see that "I was clueless" is five times as common for US writers as for British writers. Of course standard disclaimers about Ngrams apply, but the numbers are at least suggestive.
I don't think that this belongs in the body of the answer, but in looking for clues to differences in usage I found that the most common word to precede clueless in the American corpus is was, and following the path of most common following words it appears that the most common phrase containing the word is likely was clueless about how to. To me, this phrase is suggestive of a lack of knowledge that can be rectified. In the British corpus, on the other hand, the most common word to precede clueless is a; a clueless___ looks very much as if it is going to be an insult.
(To find this, I used a wildcard search to find the most common word preceding clueless in each corpus—"* clueless"—and then added wildcards to the end of the most common phrase until Ngram returned no answers. That is, "was clueless *" in American English gives "was clueless about" as most common, etc. until "was clueless about how to *" gives no results. In the British English corpus, "a clueless *" is the dead end.)