What Does TK Mean?

I was reading a post today, 10 Rule for Rewriting History, when I came across this sentence:

Since I had no idea how to fly a B-25, I initially used terms like “gas pedal [TK]” and “steering stick [TK]” as temporary placeholders.

Because I've just recently left the isolation of my word documents and ventured into the online world of writers and agents and publishing, I wondered, what does TK mean?

Of course I googled it right away, and one of the first results was The Two Words That Will Help You Write Faster.

For those who don't know, TK means "to kum" (to come). You write TK in your draft next to something that needs to be looked up or checked later, in order to avoid pausing the writing process to look it up right then. It should be a useful tool for me as I continue writing the first draft of my WIP.