Sorry. I forget that the version of English I speak is almost a dead language; no one I know talks like me any more, sentence constructions especially, and even sometimes vocabulary, but I digress.
Stet is an editing term, or, more specifically, an author's term directed at editors.
When editors would send the edited copy back to the author, if he disagreed with an edit, he would write "stet" meaning that he wished that particular word or sentence to go back to his original version. I am unsure of the complete etymology, but I believe it is derived from "Let it stand" being shortened, for efficiency.