吃醋 literally means 'to eat vinegar', but it's a cultural idiom for romantic jealousy. The expression comes from a Tang Dynasty story where Emperor Taizong offered a concubine to a minister, and the minister's wife drank vinegar (a 'poison') rather than accept the arrangement. As a separable verb (离合词), you can insert objects or measure words between 吃 and 醋, e.g. 吃他的醋 ('be jealous of him') or 吃过一次醋 ('have been jealous once').