to become bent or curved because of heat or pressure, or to make something bend or curve in this way:
bend[intransitive and transitive]
The steel pillars began to buckle.
The rails buckled under the intense heat of the fire.
if your knees or legs buckle, they become weak and bend [= give way]:
John felt his knees start to buckle.
to do something that you do not want to do because a difficult situation forces you to do it [= give in]:
do something you do not want[intransitive]
He refused to buckle.
buckle under the pressure/strain/weight
A weaker person would have buckled under the weight of criticism.
to fasten a buckle or be fastened with a buckle:
fasten[intransitive and transitive]
Amy buckled the belt around her waist.
buckle something on/up/together
Lou was buckling on his revolver.
buckle downphrasal verb
buckle down to
You'd better buckle down to some revision now.