"Gallows" is the traditional British term for this type of seat post, stemming from the similarity in shape to a "gibbet" as illustrated by @hoofhearted above.
Interestingly, an old synonym for "Gallows" is "Potence", which is French for stem. Search eBay France for potence de guidon and you will find handlebar stems a plenty!
Looking into the Cabe archives, this name has popped up in the past.
Just be thankful that you don't have to talk of mounting your saddle on a 'Tyburn Tree'.
English language, often befuddling, even to initiates!