The spelling of the word "curest" can be a bit confusing since it is not commonly used in modern English. However, the IPA phonetic transcription can shed light on this. "Curest" is pronounced /ˈkjʊrɪst/, with the stress on the first syllable. It is the second person singular form of the verb "cure", which means to heal or treat. In other words, "curest" is a somewhat archaic way of saying "you cure" or "you are curing."
The word "curest" is derived from the Middle English word "cure(n)", which in turn comes from the Old French word "curer". The Old French word was derived from the Latin word "cura", meaning "care" or "cure". Over time, "cure(n)" and its variations evolved in English to become "curest".