There is no Full form of ARMY as such. An ARMY can be defined as a land force or a ground force that fights primarily on land. In broad sense, it is the land-based service branch, military branch, or armed service of a state or nation. ARMY may also include assets related to aviation by possessing an ARMY aviation component. The word ARMY may also mean a field army within a national military force.
However, we can say that the Full form of Army is Alert Regular Mobility Young.