"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]
"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.
"distort, warp, bend out of shape" 1520s, bokelen "to arch the body," from 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.