In fact, in a 2009 paper in Discourse & Society, Judith Baxter and Kieran Wallace describe a particular use of it as:
the typically British idiom ‘I’m not being funny’ [used] to downplay the effect of a sensitive or non-politically correct commentThe phrase I'm not being funny but occurs five times in the 100-million-word British National Corpus (BNC) and zero times in the 425-million-word Corpus of Contemporary American English (both at corpus.byu.edu). The material in the BNC is 20+ years old, and since the phrase seems to be on the rise, I would expect it to occur more often these days. In 2008, it made a BBC list of 20 most hated clichés. There, a 'Rosie Spectacle' comments that it's "usually followed by a highly irritating and officious remark." Let's see if that's true.
All the BNC examples come from the 17.8-million-word spoken part of the corpus:
- I 'm not being funny but she can't stick up for herself, that girl can't
- Giles won't tell me but he definitely knows the two people that've laid her. Oh aren't they lucky gits. And I think that I 'm not being funny but I think that Jim did one.
- I 'm not being funny but I think that's actually maybe quite important,
- The contract sorry is very specific. I 'm not being funny but we're nitpicking now at the difference between [...] site instructions and V Os
- And I 'm not being funny but when Malcolm did it, we would do that [a physical recount] almost two or three days after the stock taking if there were odd counts
This relates to various things that Kate Fox discusses in Watching the English. There are the "modesty rules"--i.e. cultural rules that enforce the appearance (but not necessarily the reality) of modesty and the importance of not seeming earnest, but instead always being ready to keep things light with humo(u)r. So, you have an opinion, but the need to appear modest means that you have to avoid sounding self-important. The avoidance of earnestness means that people are always ready to assume that you're joking if you seem het up* about something. So, what do you do if you want to state an opinion? You try to disguise it as a small fact ("she can't stick up for herself"), preface it with I'm not being funny but to signal that something controversial is coming, then let the listener fill in a lot of the opinion (e.g. 'she is weak and probably deserves what she gets if she won't stick up for herself'), so that you don't have to earnestly or controversially say it.
I should say, one doesn't absolutely need the but in the phrase, but it's very often there. And we can say I'm not being funny to sincerely mean just that--for instance, as a protest when someone starts laughing after you've told them something sad. That's not the pre-emptive use--the 'let me put this negative opinion here' use--that one hears so much in the UK. That said, I think that in AmE, at least, one would be more likely in those more literal cases to say I'm being serious rather than the negated I'm not being funny.
Blogger is acting very strange these days...I hope you'll be able to post comments below!
Postscript, the next morning:
I blogged in a rush last night, which isn't the best thing for working on something pragmatic. Let me just add--the funny in I'm not being funny but can indeed (as some people have written to say) be read as the 'queer, peculiar' sense of the word. But that meaning is not unrelated to the 'humorous' meaning. It's best translated, I think, as 'I'm not trying to be difficult, but...'. But I do believe that the choice of funny in this phrase plays on this ambiguity--it's saying both 'I'm not making a joke' and 'I'm not being eccentric'. (Glad to see some comments are getting through--I know some others haven't. What's up with Blogger, eh**?)
† I belatedly found where I'd written down what BH said, so I've replaced my earlier 'the coffee is really disgusting' with the much more British understatement 'has gone downhill'. 'Has become disgusting to me' is what he meant though. This means I've also changed some further references to his statement. And, for the record, I like Costa's coffee and BH has been complaining about everyone's coffee lately...
* orig. BrE dialectal & AmE, now more common in BrE
** The eh is prevalent in Canadian English but also in my not-so-far-from-Canada AmE dialect.