Is Hush up an Idiom?

to be quiet; to get quiet; to stop talking. You talk too much. Hush up!

What does hush up mean?

: to be quiet I told him to hush up.

Is up to something an idiom?

(idiomatic) Doing something mischievous or scheming. He seems like an angel on the surface, but I can tell he's up to something.