buckle (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]

buckle (v.1)

"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)

"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.

Others are reading

Definitions of buckle
buckle (v.)
fasten with a buckle or buckles;
Synonyms: clasp
buckle (v.)
fold or collapse;
His knees buckled
Synonyms: crumple
buckle (v.)
bend out of shape, as under pressure or from heat;
The highway buckled during the heat wave
Synonyms: heave / warp
buckle (n.)
fastener that fastens together two ends of a belt or strap; often has loose prong;
buckle (n.)
a shape distorted by twisting or folding;
Synonyms: warp