buckleyoudaoicibaDictYouDict[buckle 词源字典]
buckle: [14] English acquired buckle via Old French boucla from Latin buccula ‘cheek strap of a helmet’. This was a diminutive form of Latin bucca ‘cheek’ (source of French bouche ‘mouth’), which gave English the anatomical term buccal ‘of the cheeks’ [19], and some have speculated is related to English pock. The notion of ‘fastening’ implicit in the Latin word carried through into English.

As well as ‘cheek strap’, Latin buccula meant ‘boss in the middle of a shield’. Old French boucle adopted this sense too, and created the derivative boucler, originally an adjective, meaning (of a shield) ‘having a central boss’. English borrowed this as buckler ‘small round shield’ [13]. The verb buckle was created from the English noun in the late 14th century, but the sense ‘distort’, which developed in the 16th century, comes from French boucler, which had come to mean ‘curl, bulge’.

Also from the French verb is bouclé ‘yarn with irregular loops’ [19].
=> bouclé, buckler[buckle etymology, buckle origin, 英语词源]
buckle (v.1)youdaoicibaDictYouDict
late 14c., bokelen, "to fasten with a buckle," from buckle (n.). Related: Buckled; buckling. To buckle down "apply effort, settle down," (1874) is said to be a variant of knuckle down (see knuckle).
buckle (n.)youdaoicibaDictYouDict
"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]
buckle (v.2)youdaoicibaDictYouDict
"distort, warp, bend out of shape" 1520s, bokelen "to arch the body," from Middle French boucler "to bulge," from Old French bocler "to bulge," from bocle "boss of a shield" (see buckle (n.)). Meaning "bend under strong pressure" is from 1590s (figurative from 1640s) . Related: Buckled; buckling.