A perfectly legitimate Army term
<Unass (un-ass): to get out of a vehicle, often in a great hurry.
Usage: Man, when that RPG hit my truck, I unassed it in a hurry!
Or, from a Slang website:
unass the ao: unassemble the area of operation, to leave a particular spot, widely used in the US Army
Example: What do you say we unass the ao and grab some chow?
Another example often comes when new recruits arrive at basic training and are greeted with "Unass my bus you maggots!"