Ironclad

Part of speech: noun (n.)

A naval vessel having the parts above water covered and protected by iron or steel usually in large plates closely joined and made sufficiently thick and strong to resist heavy shot.

Part of speech: adjective (a.)

1. Clad in iron; protected or covered with iron, as a vessel for naval warfare.

2. Rigorous; severe; exacting; as, an ironclad oath or pledge. [Colloq.]