Today it means: A fictional illness in which a person bearing an unrequited love coughs up flower petals until they die or their feelings are reciprocated. The disease can also be cured by a surgical procedure that removes the flowers, but this will result in them losing their feelings of affection and all memories of the person they loved. Full definition →
From Japanese 花吐き病 (hanahaki-byō), from 花 (hana, “flower”) + 吐く (haku, “to vomit”). An invention of Matsuda Naoko (松田奈緒子) for her manga Hanahaki Otome (『花吐き乙女』).
Etymology from Wiktionary, CC BY-SA 4.0.