Is "be up to" an expression? What does it mean?
"Up to" in this context is an expression that means "doing". Here, "up to" has a negative connotation because the speaker is suspicious about that the person he is talking to is doing. "Up to" can have a negative connotation, like it does here, or a neutral connotation, depending on the context it is used in. Ex: What have you been up to this weekend? (This sentence means "What did you do this weekend?" and has a neutral connotation) Ex: Are you up to something? (This sentence means "Are you doing something suspicious?" and has a negative connotation)