Why is the word "whatnot" a construction of "what" and "not"? [closed]

I've done some online etymological research on the word "whatnot", but I've been unable to figure out why it is a construction of "what" and "not".

How does the combination of the words "what" and "not" relate to the meaning of "whatnot" (simply, anything)?

I think of it as a contraction of "what[ever was] not [mentioned]" - i.e., everything else. I don't know whether that's etymologically correct, though.

Whatnot was originally a placeholder name like whatsit or thingamabob for miscellaneous trifling items. It is just something that did not have an obvious name.

It later became a word describing furniture for holding such things, typically a small stack of corner shelves. Etymonline gives 1530s for the first use and 1808 for the second.

I had always assumed that whatnot is an abbreviation of sorts for a exclamation like "My, what isn't on that list!"