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 "very", "extremely" 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 "thief", "evildoer". The meaning "extremely", though mentioned by dictionaries, appears to be a regionalism, but how come it uses the same character for "thief"?
A simple explanation might be that the sound "zei" 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。
賊, as this sentence on this page (see the leftest column): "這潘金蓮 賊 留心暗暗看着他". But for what that Shandong dialect came from, I have no idea yet.