helmet (n.)

mid-15c., perhaps a diminutive of Middle English helm (see helm (n.2)). But Barnhart and OED say from Middle French helmet (Modern French heaume), diminutive of helme "helmet," from the same Germanic source as helm (n.2). "Middle English Dictionary" points to both without making a choice. "Old English helm never became an active term in the standard vocabulary of English" [Barnhart].