I’m writing a story, and I’ve decided one of my characters has to lose an arm. The subsequent events are important to her development, but the injury itself isn’t. I haven’t decided how she loses it.
So here I am, fishing for ideas. Any suggestions?
It’s a futuristic setting. The character is an officer in the military.
A horrible horrible run-in with a nano-material loom. OSHA is currently investigating.
I just wrote a story with a similar premise. My soldier lost his arm fighting in a war. Seems the logical thing, if she’s military.
Perhaps a tragic fishing accident? 
Mauled by a manatee?
Severed by a squid?
Whalloped by a whale?
She had a run-in with a high-tension cable that snapped or came unmoored, or with an invisible jet of steam escaping a pipe through a tiny hole, under tremendous pressure. Or, she had been in a cargo containment area when a pallet(s)'s worth of stuff suddenly shifted, pinning her by the arm and crushing it…
Her boyfriend helps her stage an “accident” in the garage, and she gets fitted with TWIN BODY-POWERED DORRANCE #5 STAINLESS STEEL HOOKS!!!
In the truth is stranger than fiction department:
She got it wedged in a rock climbing accident and had to CUT IT OFF HERSELF! With a dull spork.
okay, maybe not a spork, but the real guy used a leatherman. 
She lost it in an training accident with a grenade when she was a boot instructor, knocking the dumbass trainee that dropped it out of the way. That trainee could now be important in some way, and owes a debt of gratitude.
She was in a car accident caused by a drunken Senator or other powerful politico and she stayed in the service due to his or her influence and now has a powerful piece of blackmail over them, because it never went public.
She lost the arm in a simple mission gone horribly awry, and her best friend died in front of her eyes as she lay there helpless (due to the injury.)
Ooh! Ooh!
This is what happens when you reverse the polarity of the generator on the port compression bay valve!
Thwack
She moved a major piece of equipment in zero G with a push, & then foolishly tried to stop it with a shove.
In a gravity well like Earth’s, it would have weighed 5 tons. In Earth orbit, it still massed 5 tons. Crushed the arm, amputation was needed, as were TWIN BODY-POWERED DORRANCE #5 STAINLESS STEEL HOOKS!!!
It’s an ugly planet, a bug planet, a planet hostile to life as we know it! AAAAIIEEE!!!
Crush injury. She fell from a moving heavy military carrier during the victory parade through the liberated ruins of Washington and it ran over her arm. But that didn’t stop her bringing down the sniper aiming at the President from a passing hoverplane.
Got kicked upstairs afterwards to drive a desk with the rest of the damn pen-pushers, but she can still handle a laser cannon like the best of them.
She’s kind of bitter about it. 
Misuse of an army-issue can opener.
How about she slipped while walking along the road during a blizzard, fell into the road, and got her arm ripped off by one of those big ass rotary snow-plows.
Thanks for bringing back a not pleasant scene I saw at a lake many years ago. A gal was run over by a boat and the propeller did a good job of chewing up her left arm. Everything from just above the elbow down was chopped up pretty good. Then to make matters worse, some buffoon decided he should beat the crap out of the guy that was driving the boat even though the accident wasn’t his fault. The gal jumped off the back of a different boat right into his path.
Mimicking the loss of a friend of mine: the sleeve of her uniform gets caught in the mechanics of a large motorized security gate (of the size necessary for the passage of Humvees and other large vehicles) and the arm is crushed severely above the elbow, requiring disarticulation (aka amputation) at the shoulder in order to avoid blood loss or infection which would be incompatible with the continuation of life.
(That’s the cold, clinical language used in the paperwork for the lawsuit my friend filed.)
She offered to lend a hand to precisely the wrong person.
Running out of veins and using the brachial artery to inject illicit drugs, and infarcting the arm.
(Yes, it’s one of my real-life patients).
She voulintarily gave up her arm so the tissue could be recycled to save the lives of several young soldiers under her command with otherwise fatal wounds.