Today in “Random Things you Learn from Being a Fantasy Author”:
When an annoying coworker who uses too much Office-speak advises you to “put a pin it in for now,” most people think it’s of the push-pin variety. A few people have theorized that it goes back to the World Wars, referring to putting a pin back in a grenade so you could save it for later. That sounds like an extremely rare occasion among soldiers who generally know not to activate a grenade until it’s go time.
My theory is that the expression comes from much farther back, from the humble belaying pin. Here (Fig.1) is how a belaying pin works aboard a ship during the Age of Sail (or anytime, really):
Note that the entire knot, except for being looped around the pin, is actually entirely on this side of the immovable railing. What you are looking at is a thick rope held very firmly in place by a very tight knot, which can nevertheless be instantly loosened in a hurry by pulling the pin.
Hence, putting a pin in something is a good way to tie something up for now, but keep it ready for quick release when you return to it later.
If you don’t believe me yet, it’s exactly what pirates are asking you to do when they say, “Yarrr, belay that order!” In their own seafaring way, they’re saying, “Okay, Ted, let’s put a pin in it for now.”
TL;DR I’ve spent more of the night than I should have learning about the history of naval warfare, shipbuilding, and sailing.