I’ve been told hiccups are related to a low stomach pH. What usually works for me is some variation of swallowing exhale and holding it in. Either hold your breath for 10-15 seconds and then take a decent gulp of water or just hold your breath, blow up your cheeks with air, and then gulp down the air in your mouth to your stomach.

Supposedly, the CO2 gets dissolved into your stomach fluid and forms carbonic acid to raise the pH. However based on this logic, stuff like tums/pepto/milk of magnesia or high pH foods like milk should also be effective at treating hiccups but nothings every worked as good as the breath thing for me.