Asymmetry of "not good" and "not bad"

Question

I'm not sure about this but it seems to me that "not good" always has a firm negative connotation, i.e. it is equivalent to "bad", whereas "not bad" doesn't necessarily have a firm positive connotation; it can lean towards "decent" or even "mediocre".

For example if you found a dish to be mediocre/slightly above mediocre "not bad" would be a fitting description. But if you found it to be mediocre/slightly below mediocre "not good" seems to be too strongly negative to aptly describe it.

So my question is: is my interpretation correct? And if so, why is this the case, since I'd expect these two phrases to be symmetric, as in, direct opposites.

Answer 1

You are correct. "Not good" always means "unacceptable" or, in the case of situations "worrying". In fact "not good" can come close to meaning "disasterous" when referring to situations. For instance

"Captain Smith, sir, we have struck an iceberg and the watertight bulkheads are being overtopped'". "Oh, that's not good".

On the other hand "not bad" on its own rarely means more than "acceptable" or "mediocre" unless it is spoken with a strongly rising inflection in which case it can mean "surprisingly good" or "much better than I was expecting". For example:

"How was the food in your cheap hotel?"

"Not bad, actually. I've had much worse in four star establishments"

Out of interest you should consider "it's alright" which always means "mediocre". This last one was included in the Beatles song Strawberry Fields where it was contrasted with "it's all wrong" giving the lines

"it's all wrong, that is to say I think I disagree"

and

"It's alright, that is to say I think it's not too bad".

That was very clever when it came out in the Sixties.

Answer 2

I would say that in American English, "not bad" can mean anything from "mediocre" to "very good", depending on inflection. In British English, "not bad" usually just means "good" or "very good" (so there's less of an asymmetry with "not good").

Interestingly, the American expression "not half bad" is generally more positive than "not bad", since it's more clearly perceived as deliberate understatement.