The important thing about those other phrases is that they originated in the UK where we use understatement as a form of humour, but it's important to understan

The important thing about those other phrases is that they originated in the UK where we use understatement as a form of humour, but it's important to understand the relationship between the speaker and listener and hidden shared context. E.g. your example "how was your food" ... "it was edible". If you've just been to a fairly average restaurant, that would be understood to mean it wasn't terrible but you didn't really enjoy it. If you're at a close friend's house for dinner and they served something amazing, and you replied "it was edible", it would be understood to mean that it was really good and far better than anything you could have made yourself. If you said the same thing to a mutual friend who didn't know about their cooking skills, it would be interpreted negatively. Another example would be replying to "How are you?" with "I'm not dead yet!" could mean anything without context, but most likely would be said to someone who knew your situation and that there'd been bad stuff going on, and would probably mean you were actually doing better than might be expected. As for "I could care less", I think it was probably just misheard as the negative is very weak "I could'(n) care less". Coupled with the end of "could" and start of "care" being the about the same position in the mouth, and the (n) requires the tongue to quickly block and release the air in the mouth whilst also nasalising the sound that lasts a fraction of a second, it would be easy for a lazy speaker to not make it clear and/or omit it, and then if the listener never bothered to think about the meaning, they could repeat it wrongly. The deliberately inverted meaning only really makes sense if it was followed by "but" that later got omitted. To me "I could care less, but ..." (and then just stopping) would have about the same effect as "I couldn't care less".