A military slang term that describes when two personnel are posted at a lookout or guard situation. One unit stays alert and awake watching for movement/activity (one up) while the other takes a nap or rests (one down). They alternate so as to be able to extend their alertness for a longer time. It is an unwritten rule that you must wake the resting person if you start to become tired yourself as the "One Up".

A game, the object of the game is to figure out how to play.

you have 3 choices: 2 up, 1 up 1 down, or 2 down. what you are is dependent on your hands/arms for instance if you have both your hands in the air then you are 2 up

the point of the game once you figure out how to play it is to play with other people and change what you are as nonchalantly as possible