How did 贼 come to mean "extremely"? While watching a tv show, I heard the presenter, a Beijinger, say 贼好. In that particular context, the only possible meaning was 很好.
Later I did some research and, as a matter of fact, it turns out 贼 zei2 does mean &quot;very&quot;, &quot;extremely&quot; in north-eastern dialects (东北话), and the usage has been popularized to some extent in internet speak.
However in Mandarin this character mainly translates as &quot;thief&quot;, &quot;evildoer&quot;. The meaning &quot;extremely&quot;, though mentioned by dictionaries, appears to be a regionalism, but how come it uses the same character for &quot;thief&quot;?
A simple explanation might be that the sound &quot;zei&quot; with 阳平 tone (second tone) in 东北话 can be represented in Standard Written Chinese only with this character 贼 (...and 鲗 cuttlefish?).
But I wonder if there's something else going on, or a more ancient relation between these two meanings of 贼。
Short answer:
Just for the convenience of writing, it has nothing relationship with &quot;Thief&quot;.

Detailed explaination:
Keep in mind: Some word in Chinese is just a pronounciation, it has no official word.
So, in your case, how did 贼 come to mean: extremely:
The answer is: for the convenience of writing.
Originally, people would say this orally:

这部电影zéi好看
我们家乡冬天zéi冷
那个人zéi有钱

When putting these word to paper, people would write the zéi with the commonly used character 贼.
also another examples:

我们那嘎达贼冷.
我们那疙瘩贼冷

here, 嘎达 = 疙瘩 , they don't have the meaning, just have the same pronunciation with the 东北方言. (north-Eastern Chinese dialect)  .
also:

bo leng gai, means 膝盖. (knee)


我昨天打球受伤了, 波棱盖疼
我昨天打球受伤了, 拨了盖疼


ga3 ji1 wo1: means 腋下 (underarm)


天气热, 我嘎鸡窝有点痒
天气热, 我鸽子窝有点痒
天气热, 我嘎子窝有点痒


nen4:  means very


你咋嫩好看呢?
你咋恁好看呢?

for more information, see also this.