A student of mine was very pleased to be able to catch me out with this word. I had assumed it meant "not particularly bothered", but apparently it doesn't. I had a hunch about this word so I asked four of the native speakers sitting with us what they thought. Three said they had no idea and one said she thought it meant something similar to what I had thought.
This student got me thinking; when no one knows the so-called 'correct' meaning, how can it still be considered correct? Likewise, if a language rules exists but no one follows it, is it still a rule? If an 'h' is dropped in a forest and no pedant is around to hear it, is it still wrong? Unfortunately language and language usage holds a kind of power over people and it's very easy, if you're that way inclined, to cow others into thinking they're getting it wrong when it's really rather questionable that they are.
For example, everyone knows that "Two negatives make a positive". So saying "I aint got nothing" must mean that you have something. Another example is 'literally' used to mean 'not literally'. "literally" means something actually happened, so if you say "I literally died" what you're saying is that you really died! That's impossible, because how could a dead person say that? so when someone says "the cross to Rooney was literally on a plate", listeners wonders how tableware has found it's way on to the pitch.
Or rather they don't. In fact no one ever gets confused about literally, and no one ever gets confused by double negatives; annoyed, yes, confused, no. What actually happens is some smart arse informs everyone that it is 'wrong' and then explains why. The redundancy of this would beggar believe if it wasn't for the fact it happens daily.
Is there any other area of human endeavour where we so readily assent to be told that we are 'wrong'? If you baked a cake which tasted delicious and someone told you the way you made it was wrong or, when eating soup, someone sneered at you for lacking an oyster fork or cutting the bread too thickly you would (rightly) think they were either insane or an intolerable bore. Make this kind of ridiculous comment with regards to language and everyone will nod approving and consider you to be a very sophisticated sort.
People get by using double negatives in English all the time. As with all these silly rules, they are of course countless exceptions that the pedants oddly let slide such as "not impossible" or "it's not that I don't...", "it's not like I don't want to". In AAVE double negatives can strengthen a statement such as "I didn't do nothing". They also exist in a huge number of the world's languages and no one gets confused. I have no problem criticizing language that is clearly illogical but this is just pedantry.
If we allow people to dictate language use to us, we end up with the bizarre situation such as the one I heard the other day on "In Our Time". The situation arose because the speaker used the word 'decimate' which has the prefix "dec" meaning "ten" as in December, the 10th month (blame Numa Pompilius for this) and means "destroy one in ten. Most people use it to mean "destroy" because having a word for "destroy one in ten" isn't that useful. On the show the speaker said, something like "the Romans decimated the enemy, -they literally killed one in ten of the enemy soldiers." Call me an old cynic but if you need to explain a word after you use it maybe it's time to admit that the meaning has changed and get on with your life and if you need to explain why something is wrong you probably need to think carefully about your definition of 'wrong'.