Writing advice from Neil Strauss:
Tk stands for “to come”, and it’s what you should use when you’re working on a first draft where your goal is to get all the ideas out, and you come to a point where you don’t have the exact word or quote or detail you need. Write tk, and come back to it later. Why tk and not tc? The explanation Tim Ferriss gives is that tk is a string that rarely appears in the English language, which makes it easy for you to search your document for tk later on and find all the things you need to fill in.