buck·le 
(bŭkəl)
(bŭkəl)n.
1. A clasp for fastening two ends, as of straps or a belt, in which a device attached to one of the ends is fitted or coupled to the other.
2. An ornament that resembles this clasp, such as a metal square on a shoe or hat.
3. An instance of bending, warping, or crumpling; a bend or bulge.
v. buck·led, buck·ling, buck·les
v.tr.
1. To fasten with a buckle.
2. To cause to bend, warp, or crumple.
v.intr.
Phrasal Verbs: 1. To become fastened with a buckle.
2. To bend, warp, or crumple, as under pressure or heat.
3. To give way; collapse: My knees buckled with fear.
4. To succumb, as to exhaustion or authority; give in: finally buckled under the excessive demands of the job.
buckle down
To apply oneself with determination.
buckle up
To use a safety belt, especially in an automobile.
[Middle English bokel, from Old French boucle, from Latin buccula, cheek strap of a helmet, diminutive of bucca, cheek.]