Up to no good: To be behaving in a dishonest or bad way; doing bad things or planning to do bad things. If someone wants to do something bad, or acts well on the surface, but has an ulterior motive, it can be said that he is up to no good.
As in He certainly looked as if he was up to no good.
中文释义：up to no good是不怀好意，不安好心的意思，还可以表示做坏事，淘气（指小孩子）。比如，某人平时不怎么往来，却突然表现的很热络，肯定不安好心，就可以用”up to no good”来描述他。再有，也可以用来表示熊孩子在捣蛋。
例句：Where’s that naughty boy? I’m sure he’ll be up to no good wherever he is.