not half bad
not half bad不是指「不太好」，
而是「一點也不壞」、「好極了」。
used for saying that someone or something is good, especially when they are better than you expected them to be
用來形容某人或某事很好，特別是當他們比你預期的要好時。
例：
It’s one of the cheapest printers around, and it isn’t half bad.
這是附近最便宜的印表機之一，而且還挺不賴。
對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.
它是對事物有一半「壞」的否定，因此它是一半好，表示認為事物有一半好就足夠了，已經超出預料到值得關注的好。
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.
當我用Not half bad時，意思是我說的東西是一般般的，但是比50% good好一點，也就是大約51%。雖然不是很好，但在緊要關頭也足夠好了。
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.
這就相當於說，這很好。大多數使用這個短語的人實際上並沒有停下來去衡量這個這個好與壞誰更多誰更少的時間。我們在學習英語表達時，剛開始可能以字面意義作為唯一的意思。然而，隨著時間的流逝，語言像電話遊戲一樣。有個人用了一個短語，而不理解這個短語的人(或者沒有正確地聽到這個短語)會假定它的意思。當這種情況繼續下去時，像「I could care less(字面意思是我可以在乎得更少，但實際意思和I couldn't care less 壓根就不在乎一樣)」這樣的表達就會出現。
So when you hear this phrase used, just know that the speaker is complementing something and move on.
所以如果你真的在生活中聽到這種很繞的短語，知道說話人是在補充什麼內容，然後繼續聽就行了。