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
Samantha Harvey’s novel Orbital is a sensuous, exhilarating meditation about the strangeness of life on a space station, with its mix of tedious tasks and jaw-dropping views. And: a musician who rode the rails in his youth shares the slang he...
Steve, a singer-songwriter from Rock Springs, Wyoming, shares some slang he picked up while months riding the rails and busking. Spanging refers to panhandling, from asking for “spare change.” The term bull refers to a type of security...
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.