buckle

•   verb1   

[I,T] also buckle up to fasten a buckle, or be fastened with a buckle:
?The strap buckles at the side.
2    [I] if your knees buckle, they become weak and bend
3    [I,T] to bend because of heat or pressure, or to make a substance bend in this way:
?The bridge had buckled over the years.
buckle down phrasal verb [I] informal
to start working seriously:
?You'd better buckle down or you'll never pass your exams.

 buckle 2

• noun [C]

a thing made of metal used for fastening a belt, shoe, bag etc