Do you have a dog?
I had my dog’s leash wrapped around my hand to shorten it as I approached my neighbor’s fence. My dog’s bestest friend (another dog) was on the other side of the fence, and my dog simply couldn’t wait to see him. She charged forward to greet him, smashing my hand directly into the wooden fencepost. Hurt like a sumbitch.
sorry, I have no imagination. Can’t top the llama story. But that kinda story is only good for close friends.
If you need a simple one-line answer for strangers standing impatiently at the cash reqister, how 'bout just saying you got caught by a slamming door? All your fingers got hurt, but “fortunately” only needed a one splint.
As for the OP – my favourite answer when I busted my face open (I was beyond drunk at a concert – did not want to tell that story to everyone!!) was “I don’t want to talk about it, suffice it to say – do NOT piss <my then-14 yo> daughter off!” It was funny, and kept people from asking too many questions – especially since it was right before we went to WDW on the school (band) trip!
I would suggest something similar – like I could see one of my baristas pointing to me (I am a regular – make sure if he does this, he picks a regular with a sense of humour!) and saying “when she says no water, she means no water!” 
As for it being a “guy-thing” to hit things? Wow…uhm, not always. I hit things when I get mad. Better than hitting people – things don’t have you put in jail! My daughter hits things, too. I think in general, girls probably do kick more than hit – women have lower body strength, men have upper body strength, it’s only natural.
I would suggest that your husband think about walking away from the baby when he gets frustrated, though. I know this thread isn’t about that, but let me just tell you a personal story – my daughter could be a hell of a handful when she was little and her dad would get frustrated sometimes, too. Never in his life would he hit her, and gods know he isn’t the best guy on the planet, so that’s saying something! One day, he did a similar thing – laid her on the couch beside him and hit the back of the couch – as a drummer, that effer has some strength – he knocked a picture off the wall that narrowly missed both his and our daughter’s heads. Had he walked away and hit something else, there would have been no danger to her. Just sayin’ it’s ok to get angry and it’s ok to hit things that aren’t the baby, but it’s best to hit things far away from the baby.
“You should see the other guy.”
When I broke that same bone, I had some convoluted story about saving a colony of nuns from a bunch of terrorists. That saved me from telling the truth, which was that I was beaten up by a skateboard.
“My six-month old son broke it. Yes, I’ve already contacted Don King.”
“A customer kept telling me how to make his drink. I lost control.”
“I was giving my wife her birthday present and she crossed her legs.”
Wasn’t there a movie with Richard Dreyfuss where he had a broken arm, and every time someone asked about it, he had a different story? If I recall, at the end of the movie, it turns out he broke it skateboarding.
Could he just be obtuse? “What hand??”
Or look at it in surprise: “OMG - When did *that * happen??”
That’s the sort of thing my husband would do. But he’s marginally insane…
“All I gotta say is, when they say to keep your arms and hands inside the car 'till the ride stops…you should listen”
“I was ‘beating off’ an attacker.”
I like this one! 
He was out fishing and as he cast his line a goose flew in front of him and got hooked on his lure. Rather than leave the poor goose to be pierced by a fishing lure for the rest of his life your husband kindly reeled him in (which takes a lot of effort…ask anyone who has ever caught a goose with a fishing pole!) and removed the lure but afterwards the goose bit his hand so hard he fractured a bone.
This happened with my dad and a seagull (no bone breakage, just a nasty cut where the bird bit him) but since he needs to explain a broken bone I say make the bird a goose or albatross or something.
“So there I was, masturbating like a motherfuck…”
He needs utter only one word, fairly deadpan, while looking the asker in the eye:
“Alligator.”
Hee hee! You guys are great! He’s gone to work now, and has taken a bunch of your ideas with him.
OtakuLoki, you rock. That sort of over-involved, fairytale-type story is exactly what I was looking for!
And yeah, we are going to work on finding him some other coping mechanisms…
Mine was “playing volleyball”. Not only was it 100% true, but also got a couple of oooh-er raised eyebrows from my male friends… 
“A customer came in asking for a double decaf lowfat organic mocha almond cappucino with cinnamon sprinkled on top. So I push the button and nothing. I notice the flummery agitator has become bedrazzled with latent incremental crystalline residue on our Gasmotronic NXP13 high-test cappucino synthesizer. Well, I take out my Lagstrom seven-inch gangly wrench to open it for a look when—and nobody had told me this—the solenoid switchplate hadn’t been degaussed they were still trying to figure out how to obfuscate the derivative colloidal synapses when…”
You only kick people in the toes?
My sister broke my arm: she thought I needed a push to get started down the hill, while I was not expecting one. I graduated from HS in a cast.
I can do better than one line. How about one word?
“Fisting”.
That should kill any further enquiries on the subject stone dead.
Or he could just say he hurt it moving a bookshelf.