Hiccups: A new explanation for the mysterious reflex - PMC - NIH

At its core, a hiccup is a sudden, involuntary contraction of the diaphragm — the large, dome-shaped muscle at the base of your lungs that plays a vital role in breathing. Imagine your diaphragm having a tiny, unexpected muscle spasm. When this happens, you take a quick, sharp gulp of air. But then, almost instantly, the glottis (the opening between your vocal cords) slams shut, abruptly stopping the airflow and producing the characteristic "hic" sound.

A Sudden Noisy Stopping of the Breath: Understanding the Two-Way Spellings of a Choking Sound

In modern English, hiccup is the preferred spelling in both American and British English. Using "hiccough" might be considered archaic or hyper-corrected, though it is not technically wrong. Why Does This Happen?

Why is this word connected to heat?

: An older variant that originated from a mistaken belief that the sound was related to a cough.

Expanding the stomach with trapped gas.