what does nimbus and TK mean in the sentence？
“Why didn’t you just have all of us go through the back entrance?” Then he muttered under his breath (but Miles heard him), “You nimbus.”
It was true. The school did have a back entrance, and Barkin had walked through it this morning. (Please see page TK.)

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Well, you've probably guessed "nimbus" doesn't mean a cloud here. :)  If you guess the meaning from the context, you'll be right in thinking that it means "idiot". My guess is that it's a combination of the words "nimrod" and "doofus", which both mean a foolish person.  It looks like something that the writer made up.

I had to look up TK - apparently it means "to come", referring to information that will be added later.