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
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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.