1Tough, hardy; unyielding, inflexible.
2Of a material object: having sides which are made of iron, or which are as strong as iron.
3historical Of a warship: having sides strengthened with plates of iron or steel; = "ironclad". Compare "Ironside".
Mid 17th century; earliest use found in John Gauden (c1599–1662), bishop of Worcester. From iron + sided, after Ironside.