It’s a simple six-letter word set in bold italics in the FAA’s Pilot/Controller Glossary—that Rosetta Stone-type appendix found in the back of most FAR/AIM manuals.
The glossary provides a common tongue between the flight deck and air traffic control, and bold italics indicate terms most frequently used in pilot/controller communications. As a pilot in training, you should be familiar with the entire glossary, but the bold items—similar to bold items on many airplane checklists—should be committed to memory.
It’s important to note that “unable” doesn’t mean “impossible.” It means that you have chosen not to do whatever it is you’ve been asked to do for your own reason(s). After all, a controller doesn’t really know what you and your airplane are capable of, and sometimes unintentionally asks you to do things that are unsafe, or even just simply uncomfortable. As pilot in command it is both your right and your duty to indicate that you are outside your comfort zone, and that you are, well, unable to comply.
For example:
The beauty of “unable” is that it ends the conversation. No further explanation or justification is required (although if time allows, at your option, you can explain and offer an alternative course of action).
So, enable yourself. Be ready to use the most powerful word in aviation.