1
general::
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.
American Heritage Idioms
2
general::
Phrase(s): buckle up [and] belt up
to buckle one’s seat belt, as in a car or plane. • Please buckle up so our flight can begin. • I wish you would obey the law and belt up., Phrase(s): buckle someone or something up
to attach someone or something securely with straps that buckle together. (This emphasizes the completeness and secureness of the act.) • Buckle the children up before we leave. • Buckle up your shoes.
McGrawhill's American Idioms And Phrasal Verbs