Buckle

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. The buckle allows you to adjust the tightness of your belt.
2. An instance of bending, warping, or crumpling. The pavement buckled due to the excessive heat.
3. To succumb, as to exhaustion or authority. He buckled under the excessive demands of the job.
4. To give way; collapse. His knees buckled with fear.