means The hollow, muscular organ between the esophagus and intestines where food is churned and broken down by acid, and by extension the appetite or nerve to tolerate something ('couldn't stomach it').

from From Greek 'stomachos,' which first meant a throat or gullet — literally the 'mouth' of a part, from 'stoma,' mouth — and only later slid down the body to name the stomach itself. It traveled through Latin 'stomachus' (which also carried a sense of pride, taste, and irritation — hence to 'stomach' an insult) and into English via Old French. That older emotional cargo is why we still 'have no stomach' for things we can't bear.