buckler n.

"spiked metal ring for holding a belt, etc.," c. 1300, bukel, from Old French bocle "boss (of a shield)," then "shield," then by further extension "buckle, metal ring," (12c., Modern French boucle), from Latin buccula "cheek strap of a helmet," in Late Latin "boss of a shield," diminutive of bucca "cheek" (see bouche).

Boucle in the middle ages had the double sense of a "shield's boss" and "a ring"; the last sense has alone survived, and it metaph. developed in the boucle de cheveux, ringlets. [Kitchin]

Middle English sheld, "frame or rounded plate of wood, metal, etc., carried by an warrior on the arm or in the hand as defense," from Old English scield, scild "shield; protector, defender," originally "board," from Proto-Germanic *skelduz (source also of Old Norse skjöldr, Old Saxon skild, Middle Dutch scilt, Dutch schild, German Schild, Gothic skildus), from *skel- "divide, split, separate," from PIE root *skel- (1) "to cut."

The IE sense evolution of that proposal is uncertain; the ancient notion is perhaps a flat piece of wood made by splitting a log, but Boutkan writes, "it seems more probable to me that the word designated a means of protection, i.e. a separation between the fighter and the enemy."

Shield usually meant a larger defensive device, covering much of the body, as opposed to a buckler. Shield volcano (1911) translates German Schildvulkan (1910). The plate tectonics sense of shield as "large, stable mass of Achaean rock forming a continental nucleus" is by 1906, translating Suess (1888).

also swash-buckler, 1550s, "blustering, swaggering fighting man" (earlier simply swash, 1540s), from swash "fall of a blow" (see swash) + buckler "shield." The original sense seems to have been "one who makes menacing noises by striking his or an opponent's shield."