helmet

Helmet comes from Middle English helmet, borrowed from Old French helmet or heaumet, a diminutive of helme. That French word traces back to a Germanic source, the same root that gave Old English its own word, helm.

Since entering English in the 15th century, helmet has largely taken over from the older word helm as the everyday term for head armor.

It's built from helm plus the diminutive ending -et, so it's essentially a 'small helm' — helm being the older English word it eventually replaced.