In the tag '#gnu terry pratchett', what does the 'gnu' stand for?
I'm going to copy paste part of my answer because honestly, I always have to look it up to remember exactly what each letter stands for, and other people before me have explained it better, so:
In Terry Pratchett's Going Postal, the operators of the clacks tower use “GNU” as a code to mean the message should be passed onto every tower. An article on wiki-Lspace says that “The G in the code means to send it on the U at the end means to turn the message around at the end of the line and the N means not logged.”
I could also go into more detail about the significance of GNU in the Discworld book Going Postal, but then I'd have to venture into minor spoilery territory, and it's not necessary to understand GNU either.
Since Terry Pratchett's death a few years ago, the phrase has become of common usage among fans as a way to both mourn him and remember him. It's a bit redundant to use both his name and the GNU versions in my posts, but I do it for sentimental reasons and I also use X-Kit so it only takes a click to enter all the generic Discworld tags in a post.