One of the mysteries of the English language finally explained.
1(An epithet of) Edmund II, king of England (reigned 1016).
2Generally (A name for) a person of great physical or mental strength. Also in plural in same sense. Now rare.
3(An epithet of) Oliver Cromwell. Also preceded by old.
4A member of Oliver Cromwell's cavalry; a Puritan soldier. Also in extended use. Now historical.
5Chiefly with lower-case initial. A warship plated with iron; = "ironclad". Now historical.
Old English; earliest use found in The Anglo-Saxon Chronicle. From iron + side.
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