Learn English – a “temporary mental apparition”?

meaning

From The Ransom of Red Chief by O. Henry:

It was, as Bill afterward expressed it, "during a moment of temporary mental apparition;" but we didn't find that out till later.

What is meant by it?

Best Answer

For context, the full first paragraph is:

It looked like a good thing: but wait till I tell you. We were down South, in Alabama -- Bill Driscoll and myself -- when this kidnapping idea struck us. It was, as Bill afterward expressed it, "during a moment of temporary mental apparition"; but we didn't find that out till later.

According to this copy of the story, here Bill mistakenly uses apparition for aberration--a disorder of the mind. There are several definitions of aberration, but the relevant one is:

mental irregularity or disorder, especially of a minor or temporary nature; lapse from a sound mental state.

So the temporary mental apparition was supposed to be temporary mental aberration--that is, a moment of temporary insanity.