Deboard does have a real meaning, but it is not the same thing as deplane.
To deboard is to leave the aircraft you just boarded at the same airport (usually the same gate).
In other words, it simply means to un-do the processing of boarding. It's like going back in time.
When there is a mechanical problem before boarding begins, normally boarding is delayed until the mechanical problem is fixed. If the mechanical problem is discovered after everyone has boarded, and you wish to leave the plane to make other arrangements, you must de-board. Otherwise, the airline's computer will think you're going to two different places at the same time.
On some airlines the gate agent may be able to de-board you just by typing on the computer. On UA, you have to put your boarding pass in the boarding pass reader just like when you boarded in the first place.
I wonder if there is a term "reboard" -- as in: board the plane, mechanical problem announced, deboard, mechanical problem fixed immediately, and then ... reboard!
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"There's no sadder sight in this world than that of a football player trying to think." -- Daria