What is a w[h]ister - clister ??

That's my revised question.
I started with the question,
what is a wisty castor?, after reading it in
"Friday's Child," one of the neo-Regency revival novels by
20th-century author Georgette Heyer.

When I looked up "wisty castor," the explanations suggested
that this latter is an adaptation of
"whister / wister - clister."

So, I am none the wiser.

I mean, I KNOW what they are speaking of , which is,
a pow in the kisser,
or a punch to the face/jaw/ear.

In the book, two gentlemen exchange words, and come to blows,
and one of them gives the other a "wisty castor."
That's plain enough.
So that isn't my question.

More deliberately put,
whence comes the phrase "whister - clister" ?

Many thanks in advance.