The Collaborative International Dictionary of English v.0.48:
Buckle \Buc"kle\, v. t. [imp. & p. p. Buckled; p. pr. & vb. n.
Buckling.] [OE. boclen, F. boucler. See Buckle, n.]
1. To fasten or confine with a buckle or buckles; as, to
buckle a harness.
2. To bend; to cause to kink, or to become distorted.
3. To prepare for action; to apply with vigor and
earnestness; -- formerly, generally used reflexively, but
by mid 20th century, usually used with down; -- as, the
programmers buckled down and worked late hours to finish
the project in time for the promised delivery date.
[1913 Webster +PJC]
Cartwright buckled himself to the employment.
4. To join in marriage. [Scot.] --Sir W. Scott.