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.
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   2. To bend; to cause to kink, or to become distorted.
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   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.
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            Cartwright buckled himself to the employment.
                                                  --Fuller.
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   4. To join in marriage. [Scot.] --Sir W. Scott.
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