justonce breaker / justonce bar / justonce stop / etc.: Engineering jargon. A justonce in any of its forms – be it the hardline switch or insertable bus-bar that bypasses circuit overload protection, the stop valve that seals off pressure-relief valves, or the disabling of thermal trips is the manual override which permits equipment to continue operating even when doing so will result in damage to equipment, personnel, or the surrounding area – even, in its most common military applications, to the entire containing vessel.
The justonce, naturally, is intended to be used only in situations where the risks and consequences of having a given piece of equipment out of action are outweigh the risks and consequences of it failing catastrophically.
The unusual name for these devices refers to a cynical saying common in the Imperial Military Service: “Doesn’t matter what they’ve got; you can’t explode twice.”
See also: damnfool switch.
– A Star Traveler’s Dictionary