From Longman Dictionary of Contemporary Englishbe up to no goodbe up to no goodinformalDISHONEST to be doing or planning something wrong or dishonest Those guys look like they’re up to no good. → good
Examples from the Corpus
be up to no good• Those guys look like they're up to no good.• She knew that her brother was up to no good but she didn't tell anyone.• If you ask me, that husband of hers is up to no good.• Anyone waiting around on street corners at night must be up to no good. 