Buckle down

= start to work seriously

We'll buckle down to start the project.

buckle: a flat, typically rectangular frame with a hinged pin, used for joining the ends of a belt or strap.
(as a verb) fasten or decorate with a buckle.

As you know, buckle means a seat belt. "buckle up" means just putting on a seat belt as it is.

pv, 14-06-05 19:32, 14-06-05 19:32, Yosei