Helmet
late Middle English: from Old French, diminutive of helme, of Germanic origin; related to helm2.
wiktionary
From Middle English helmet, helmett, a borrowing from Old French helmet, heaumet, a diminutive of helme (Modern French heaume), equivalent to helm + -et. The Old French is itself of Germanic origin (whence Old English helm). Used in English since the 15th century, it has largely displaced helm as the general word.
etymonline
helmet (n.)
mid-15c., perhaps a diminutive of Middle English helm (see helm (n.2)). But some sources suggest Old French heaumet (Modern French heaume), a French diminutive of helme "helmet," from the same Germanic source as helm (n.2); Barnhart writes: "Old English helm never became an active term in the standard vocabulary of English."
