It's taken a while for me to find this out

Hiccups: The first air-breathing fish and amphibians extracted oxygen using gills when in the water and primitive lungs when on land — and to do so, they had to be able to close the glottis, or entryway to the lungs, when underwater. We descendants of these animals were left with vestiges of their history, including the hiccup. In hiccupping, we use ancient muscles to quickly close the glottis while sucking in (albeit air, not water). One of the reasons it is so difficult to stop hiccupping is that the entire process is controlled by a part of our brain that evolved long before consciousness, and so try as you might, you cannot think hiccups away.

source: smithsonianmag.com