Dic/ a usage/ up to something

─ occupied with, engaged in, as in:

• What have you been up to lately.

 . This usage can mean "devising" or "scheming" as in:

• We knew those two were up to something.

 . It also appears in up to no good, meaning "occupied with or devising something harmful," as in:

• I'm sure those kids are up to no good.

─ (informal) doing something, esp. something bad.

• What's she up to?
• What've you been up to?
• I'm sure he's up to to no good (= doing something bad).

... The American Heritage® Dictionary of Idioms, OALD