AOG is a bad word in aviation nobody likes to hear or read anywhere. It is an abbreviation and stands for “Aircraft on Ground” which indicates there is an issue with the aircraft serious enough to prevent it from flying. And even though it is an acronym, it is used like a (nasty) word as in “I am afraid, the aircraft is AOG” (usually followed by gasps and “awwwww”).
The whole thing usually comes with a sense of urgency to acquire parts quickly in order to put the aircraft back into service therefore avoiding further delays or cancellations. Parts guys usually have big red and white stickers that say “AOG” on them to slap on the side of the box and packages often get shipped “counter to counter” with the airlines which is pretty much the fastest way. It’s expensive but compared to an aircraft not making money, people getting delayed, or worse like in my job, fire not getting put out, it is often deemed worth it.
Working as a mechanic at a small flight school of course I have often seen the “emergency” turning out to be not quite as urgent as first suspected. Although the failure of a coffee machine or the only working microwave might just qualify as an emergency situation. After all, NOBODY wants to eat a cold bratwurst …