If you hiccup 100 times, you'll die

In the past, education wasn't widespread and only a small number of samurai families were able to count, so the number 100 here means an innumerable number.

In addition, in the past, medical care was not developed and the causes of death were often unclear, so the phenomenon of hiccups was seen as mysterious, incomprehensible, and scary. Therefore, hiccups are a sign of a fatal illness.

Verity:

We're not convinced that "only a small number of samurai families were able to count". Like any culture, Japanese people have been preoccupied with simple methods of counting for thousands of years. Being able to count is an indispensable skill. (Visiting a foreign country where you don't know the language? Learn the words of the first ten numbers and you'll have a significant amount of useful vocabulary.)

We're also rather puzzled by the "innumerable" number of hiccups before the subject dies.

The current world record length for hiccups is 68 years. American Charles Osborne had intractable hiccups from 1922 to 1990, and was entered in the Guinness World Records as the man with the longest attack of hiccups, an estimated 430 million hiccups. He died 1 May 1991 at the age of 96.

So the good news is that hiccupping could be a harbinger of longevity!

The bad news is that it could also be a sign of damage to, or irritation of, the vagus nerves or phrenic nerves. These nerves supply the diaphragm muscle. Factors that may damage or irritate these nerves include a tumour, cyst or growth on the thyroid gland in your neck, pneumonia, pleurisy or one of many other life-threatening nasties.

But if you have such serious condition, you're most probably aware of it already from other symptoms.

The UK's National Health Service advice to stop or prevent hiccups:

If your hiccups persist more than 48 hours or if they're so severe that they cause issues with eating, sleeping or breathing, then see a doctor.