be up to no good
to be doing or planning something wrong or dishonest

[TahlilGaran] Dictionary of Contemporary English ▲

up to no good
doing something bad.
In movies or on television, a man who pulls out a cigar is up to no good.

[TahlilGaran] English Idioms Dictionary ▲

up to no good
doing something bad
The boys were up to no good after school.

[TahlilGaran] English Idioms Dictionary ▲

up to no good
adv. phr. Intending to do something bad; perpetrating an illicit act.
We could tell from the look on Dennis the Menace's face that he was once again up to no good.

[TahlilGaran] English Idioms Dictionary ▲