Boom Holder Thingy ?
Just to round out Paul''s excellent response, gallows came into useage when booms were solid wood and so way heavier than they are today. (Booms were intentionally heavy in the days before boom vangs to help keep the boom from rising in a breeze.) It was a time when booms extended aft well past the transom. Gallows were initially used in conjuction with gaff rigs and before winches. Toping lifts were a real pain in the butt on gaffers. Lifts existed but they were on both sides of the sail and had to be slacked and tensioned at just the right time.
One point that I disagree with in Paul''s response, rarely did traditional watercraft have the gallows high enough to walk under. Even on fairly large traditional water craft the booms crossed the deck quite low and needed to be ducked under. Because the booms typically extended past the aft end of the cockpit the gallows was generally aft of the cockpit and it was no problem. On our 1939 Stadel cutter the gallows was no more than about 4 feet in height.
I found that sailing with a set of gallows, if you actually used them, was a real pain in the butt with little real advantage over a good topping lift and a clip on bridle when at anchor.
Gallows built in the early to mid 20th century generally were constructed using tubing, a base and a neat little ''patent'' fitting at the top. They were not that sturdy that I would ever suggest clipping a harness onto one which is not to say that you could not build one that is that sturdy but it would look more like the arches that you see today.