Gastroesophageal reflux disease (GERD) with associated symptoms: This is the most likely diagnosis because GERD can cause hiccups due to irritation of the esophagus and stomach, and diarrhea can be a side effect of certain medications used to treat GERD or due to an underlying condition that also causes GERD.

Irritable bowel syndrome (IBS): IBS can cause diarrhea and may also contribute to GERD symptoms due to increased abdominal pressure and altered gut motility. Hiccups could be a separate, unrelated symptom.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause diarrhea and may also lead to GERD symptoms due to inflammation and altered gut motility. Hiccups are less commonly associated but could occur due to severe abdominal distress.
Gastritis or peptic ulcer disease: These conditions can cause GERD symptoms and may lead to hiccups due to stomach irritation. Diarrhea could be a side effect of medications or an indication of a more severe underlying condition.

Pneumonia or other pulmonary infections: Although less likely, pneumonia can cause hiccups and, in severe cases, lead to gastrointestinal symptoms like diarrhea due to systemic illness. Missing this diagnosis could be fatal.
Cardiac conditions: Certain cardiac conditions, such as myocardial infarction, can present with atypical symptoms including GERD-like chest pain, hiccups, and even gastrointestinal symptoms due to reduced cardiac output. These conditions are life-threatening and must not be missed.
Gastrointestinal obstruction or perforation: These are emergency conditions that can cause severe abdominal pain, vomiting, and potentially diarrhea. Hiccups could be a symptom of the underlying distress. Missing these diagnoses can lead to severe morbidity or mortality.

Eosinophilic esophagitis: A rare condition characterized by an eosinophilic infiltration of the esophagus, which can cause GERD-like symptoms, dysphagia, and potentially hiccups. Diarrhea would be an uncommon symptom but could occur in the context of a broader eosinophilic gastrointestinal disorder.
Tumor or cancer of the stomach or esophagus: Although rare, tumors can cause a variety of gastrointestinal symptoms including GERD, diarrhea (due to malabsorption or obstruction), and hiccups (due to irritation or obstruction of the stomach or esophagus).