X-Clacks-Overhead is a non-standardised HTTP header based upon the fictional work of the late, great, Sir Terry Pratchett.

In Terry Pratchett's science-fantasy Discworld series, "The Clacks" is a network infrastructure of Semaphore Towers, that operate in a similar fashion to telegraph - named "Clacks" because of the clicking sound the system makes as signals send.

In Sir Terry's novel "Going Postal", the story explains that the inventor of the Clacks - a man named Robert Dearheart, lost his only son in a suspicious workplace accident, and in order to keep the memory of his son alive, he transmitted his son's name as a special operational signal through the Clacks to forever preserve his memory:

GNU John Dearheart

G: Send the message onto the next Clacks Tower.
N: Do not log the message.
U: At the end of the line, return the message.