Buckle–up

a clasp consisting of a rectangular or curved rim with one or more movable tongues, fixed to one end of a belt or strap, used for fastening to the other end of the same strap or to another strap.
any similar contrivance used for such purposes.
an ornament of metal, beads, etc., of similar appearance.
a bend, bulge, or kink, as in a board or saw blade.
to fasten with a buckle or buckles:
Buckle your seat belt.
to shrivel, by applying heat or pressure; bend; curl.
to prepare (oneself) for action; apply (oneself) vigorously to something.
to bend, warp, or cause to give way suddenly, as with heat or pressure.
to close or fasten with a buckle:
Grandmother always wore shoes that buckled.
to prepare oneself or apply oneself:
The student buckled to the lesson.
to bend, warp, bulge, or collapse:
The bridge buckled in the storm.
to yield, surrender, or give way to another (often followed by under):
She refused to take the medicine, but buckled under when the doctor told her to.
buckle down, to set to work with vigor; concentrate on one’s work:
He was by nature a daydreamer and found it hard to buckle down.
buckle up, to fasten one’s belt, seat belt, or buckles:
She won’t start the car until we’ve all buckled up.
Contemporary Examples

The New Unsafe Sex Tracy Quan July 27, 2009

Historical Examples

Greenmantle John Buchan
The Life and Letters of Walter H. Page, Volume II Burton J. Hendrick
Trapped by Malays George Manville Fenn

noun
a clasp for fastening together two loose ends, esp of a belt or strap, usually consisting of a frame with an attached movable prong
an ornamental representation of a buckle, as on a shoe
a kink, bulge, or other distortion: a buckle in a railway track
verb
to fasten or be fastened with a buckle
to bend or cause to bend out of shape, esp as a result of pressure or heat
n.

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]

v.
Fasten a seat belt, as in All the children must learn to buckle up as soon as they get in a car. This term came into wide use in the second half of the 1900s, when seat belts became mandatory automobile equipment. Earlier they had been used mainly in airplanes.