Up and at ’em is a colloquial expression, motivating people to get busy, get up and start working, similar to the phrase “pitter patter let’s get at er”.
It can also be used in a military environment, for telling soldiers to get up and fight.
While “up and at ’em” would appear in various contexts in the 1800’s, the phrase would start seeing widespread usage in World War 1, when trench warfare meant that soldiers were being holed up in muddy pits.
“Up and at ’em” would be shouted by commissars and officers telling soldiers to get up from the trench and at the enemy.
“Up and at ’em” would be used in later wars such as World War 2, where US soldiers aptly named a nuclear bomb carrier as “up and atom”; a name which would reemerge in the Fallout videogame series.
“Up and at ’em” was first defined on Urban Dictionary in 2004.