So what injury could possibly hurt when you laugh? I thought this was a CS question rather than GQ; since it also explores the expression’s cultural background, not just a simple matter of physiology.
In my short list of experiences, a c-section and a frontal frenulectomy (removal of gum to correct a gap between the front teeth) are both serious non-laughing matters.
Maybe some brain aneurysm but most likely nothing could be so specific as to only include laughter but not other vigorous vocalizations.
But of course, the phrase comes from the idea that the question “does it hurt?” is laughably obvious.
Cracked ribs
I once pulled a muscle laughing too hard. It wasn’t serious or long-lasting, but it hurt like heck for a day or so.
That hurts for ANYTHING other than sitting perfectly still, maybe. Certainly not ONLY when you laugh.
I’ve cracked enough ribs to know.
It hurt to laugh for about a week after my gallbladder removal (by endoscopy). Also hurt to generally breathe deeply or laugh when I pulled some little weird muscle in my back, behind my ribcage. Some lateral breathing muscle we’re normally completely unawares of. Also, airway stricture.
Most any abdominal or chest surgery will cause that symptom. However, it’s good for you to laugh. You won’t really rip open your stitches; it just feels that way.
IANAD but have had numerous surgeries.
This is more medical than cultural, so I’m going to zip it over to IMHO.
Yep Gallbladder removal for me too… & my husband and Mother-in-law tried to kill me with hilarity 
When I was 30 I cracked a few ribs during a mountain biking fall. I could not laugh, cough or sneeze for almost three weeks without pain and discomfort.
or run, or walk, or lay down, or sleep, or eat, or shower, or drive. Literally the only thing that wasn’t painful was sitting very still.
Yep. The first week was the worst, and the first 2-3 days almost intolerable. I recuperated fairly quickly, but my body didn’t feel 100% normal for quite a while.
Oh, and you forgot one - sex.
When I used to get bronchitis for a week or two, the continual strain of breathing would sometimes fatigue the relevant muscles so much that they would become sore like any other overused muscle, so that laughing would be moderately painful for a few days even after the bronchitis had eased.
Your cracked ribs have evidently been worse than mine, maybe mine were just bruised.
In my case it hurt whenever something tougher than a cotton T touched the area, and any activity which involved quick movement of the ribs: laughing, coughing, sneezing. I could do other things so long as I was careful about them; I could yell or sing just fine so long as I was careful to breathe slowly.
I remember feeling so grateful that the spot was low enough that my bra didn’t touch it. The spot was on the front left, so it didn’t get touched as much as if it had been on the back or the side.
Pleurisy. Infection of the outer lining of the lungs. Hurts like blazes when you laugh, or even breathe deeply.
(So I had pleurisy…and was watching The Addams Family, the movie with Julia and Huston. Kept having to laugh. Must…not…laugh! Har! OW!)
I suppose it depends which of your muscles react when you laugh (i.e., how you laugh).
I’ve got sciatica at the moment, mostly felt in my right hip and leg, but somehow abdominal muscles are involved, because I have to be careful coughing, sneezing and some belly laughs. Went to the panto last Saturday and there were moments there that were uncomfortable (oh yes they were!). But a mild titter doesn’t hurt at all.
I had bruised ribs a couple of years ago, and anything involving breathing deeply would hurt. (Sneezing was especially painful.)
I’ve had bronchitis where normal breathing was okay, but laughing would make me start a coughing fit, which, of course, hurt like hell.
In 7th grade I caught a flying baseball bat in the mouth. It put a hole through my skin about a half inch under my bottom lip.
While the stitches were healing it up and it was scabbing over any time my friends made me laugh it stretched that skin and ripped the thing open painfully.