buckle (n.) Look up buckle at Dictionary.com
"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.1) Look up buckle at Dictionary.com
"to fasten with a buckle," late 14c., bokelen, from buckle (n.). Meaning "prepare for action of any kind" (1560s) probably is a metaphor from buckling on armor before battle. Related: Buckled; buckling.
buckle (v.2) Look up buckle at Dictionary.com
"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 "to bend under strong pressure" is from 1590s (figurative from 1640s) . Related: Buckled; buckling.