[Jtsummers]

Exceptional means "unusual" (by the dictionary definition). In theory, exceptions should be thrown in in exceptional (rare, unusual) circumstances. Things like running out of memory, or dealing with random poorly constructed data inputs are reasonable circumstances to use exceptions. However, if your input is consistently mangled, an exception may not be the appropriate way to handle it since it becomes a normal thing (if for no other reason than performance) depending on how you want to handle the problem and whether that performance cost is worth it.

[talaketu]

The primary definition (OED) is "forming an exception", which is why I think this cliche is a tautology ("exceptions are for forming an exception") that does nothing to guide me on whether an exception is appropriate in a given case.

Btw, we don't say "exceptions are for infrequent conditions", because that's not what they're for.

I quite like the etymological "taken out", because it carries a notion of special handling - a control flow aspect that is the main point of using them.