I've a very dim memory of having heard about it years ago (more than a decades), from an article of Cory Doctorow, and in my mind, he was the one who came up with the idea (and chose the letters TK).

But I can be wrong (maybe it's not from Doctorow, maybe the article did not even claim the paternity of coming up with TK but it was me badly understanding it, ...)

TK is a very standard term, see William Safire's usage in this 1996 NY Times article: https://www.nytimes.com/1996/10/06/magazine/of-hacks-and-tk....