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
Over the centuries, the meaning of happiness has traveled a long way. Today we speak of the pursuit of happiness, but it used to be that the word happiness suggested something that occurs only by chance–something that simply… happens. Plus, the joys...
The slang term boondocking refers to camping off-grid in an RV without a hookup to water, electrical power, or a sewer. The -docking element in the word also gave rise to Crackerdocking, which is “boondocking in the parking lot of a Cracker Barrel”...
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.