Up and Atom/Adam

This phrase appears in YOU'RE A GOOD MAN, CHARLIE BROWN (which I am directing in a few months) and I was wondering if anyone knew

A) Which is correct - Up and Atom or Adam

and

B ) Where did it come from?

Thank you in advance for any help you can be!
~Barry

Up and at 'em. I.e., Get up and go after them, get going, rise and shine.

Originally the saying was:
"Up Guards and at 'em!" - presumably one of the Guards regiments.
We seem to have lost the first bit in modern day speech.