unass v. to dismount or disembark (a vehicle); to get off of (something); to unseat (someone); to leave (somewhere). Editorial Note: This term dates back to at least the 1960s and the Vietnam War. It is especially associated with the military, from where it has spread to politics and aeronautics. (source: Double-Tongued Dictionary)
Further reading
When she was a child, Chris from Santa Rosa, California, misunderstood the definition of the word vehicle—with amusing results. This is part of a complete episode.
Related
What’s it like to hike the Pacific Crest Trail all the way from Mexico to Canada? You’ll end up with sore muscles and blisters, and great stories to tell. Along the way, you’ll also pick up some slang, like NoBo, SoBo, Yo-yo and...
This term was in use in the Army in the ‘50’s. I first heard it in 1955, when its meaning was “get up off of” as in: “Un-ass that chair!/rack!/bench!” etc.