up to (2)

(verb) to be doing something suspicious

Example Sentence:

The policeman asked the criminal what he was up to, because the criminal was covered in blood.

Mom: You kids are very quiet in there. What are you up to?
Kids: Nothing!

Similar idioms:

1. turn to
2. blow one’s lid
3. bad blood
4. hot under the collar
5. blood is thicker than water
6. out of nowhere
7. out of the blue