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.)

Question

Answer

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.

ご自宅で快適に語学を学べるチャンスをお見逃しなく。経験豊富な語学講師陣の中からお選びいただき、今すぐ最初のレッスンにお申し込みください！