1The heartwood of the oak.
2figurative A person with a strong, courageous nature, especially a brave and loyal soldier or sailor; (also) a courageous or valorous spirit. Frequently in plural Compare "heart". Now somewhat archaic.
3In plural form Hearts of Oak. (The members of) an Irish non-sectarian organization which protested, often violently, against taxes for the building of new roads, and tithes exacted by the Church of Ireland. Compare Oakboy . historical in later use.
Late Middle English. From heart + of + oak.