not half bad
not half bad不是指“不太好”，
used for saying that someone or something is good, especially when they are better than you expected them to be
It’s one of the cheapest printers around, and it isn’t half bad.
对not half bad的理解
It is a negation of thing being half 'bad', thus it is half-good, and the colloquialism grants that the half-good part quality of the thing is sufficient, interesting, or surprising enough to warrant attention.
When I use Not half bad, I mean that what I'm talking about is so-so, but a little bit better than 50% good, namely, about 51%. Not even pretty good, but good enough in a pinch.
当我用Not half bad时，意思是我说的东西是一般般的，但是比50% good好一点，也就是大约51%。虽然不是很好，但在紧要关头也足够好了。
Saying this is the equivalent of saying, That's pretty good. Most people that use this phrase do not actually stop to weigh out whether or not the subject is actually more good than it is bad. As with most expressions, it is likely that it started with the literal meaning as the only meaning. However, time makes language like a game of telephone. One person uses a phrase, and someone that doesn't understand the phrase (or didn't hear it properly) assumes the meaning. When this continues, expressions like, I could care less, are born.
这就相当于说，这很好。大多数使用这个短语的人实际上并没有停下来去衡量这个这个好与坏谁更多谁更少的时间。我们在学习英语表达时，刚开始可能以字面意义作为唯一的意思。然而，随着时间的流逝，语言像电话游戏一样。有个人用了一个短语，而不理解这个短语的人(或者没有正确地听到这个短语)会假定它的意思。当这种情况继续下去时，像“I could care less（字面意思是我可以在乎得更少，但实际意思和I couldn't care less 压根就不在乎一样）”这样的表达就会出现。
So when you hear this phrase used, just know that the speaker is complementing something and move on.