Definition
To behave in a dishonest or mischievous way; to be engaged in activities that are not good or are likely to cause trouble.
The group of teenagers was loitering suspiciously on the street corner, looking like they were up to no good.
He's always sneaking around; I suspect he's up to no good.
She found her son sneaking out of the house in the middle of the night; he must be up to no good.