ironclad

nouns

a wooden warship of the 19th century that is plated with iron or steel armor
is a type of (narrower)
a government ship that is available for waging war

adjectives

sheathed in iron plates for protection
is similar to
enclosed in a protective covering; sometimes used in combination
"his sheathed sword"; "the cat's sheathed claws"; "a ship's bottom sheathed in copper"; "copper-sheathed"
inflexibly entrenched and unchangeable
"brassbound traditions"; "brassbound party loyalists"; "an ironclad rule"