From Longman Dictionary of Contemporary Englishbe on firebe on fireliteraryHURT/CAUSE PAIN a part of your body that is on fire feels very painful → fire
Examples from the Corpus
be on fire• Here, some bright spark thought Windsor Castle was on fire and called the fire brigade!• He stabbed his chips like a killer and poured beer down his throat as though his guts were on fire.• Miguel opened his door and lunged as though his head were on fire.• They went willingly enough, but their hearts were on fire with jealousy.• It was impossible that these two whose hearts were on fire should be kept apart.• Before long the neighboring houses were on fire too.• They make the skin crawl like it is on fire, even as it is bathed in sweat.• The Steam Laundry was on fire.• If her cheeks had been hot when he arrived, they were on fire now.• Large areas of the forest are reported to be on fire. 