反胃 is a separable verb (离合词) meaning 'to feel nauseous' or 'to have an upset stomach'. It's formed from 反 (reverse) + 胃 (stomach). In practice, you can split 反 and 胃 to insert 了, 过, or measure words: 反了胃 (got nauseated), 反过两次胃 (had nausea twice). The word also carries a strong figurative sense — you can say something 让人反胃 ('makes people sick') to express moral disgust, similar to being 'sickened' by something in English.