What is "to buckle to"

Buckle \Buc"kle\ (b[u^]k"k'l), v. i.

1. To bend permanently; to become distorted; to bow; to curl; to kink.

   Buckled with the heat of the fire like parchment.
   --Pepys.

2. To bend out of a true vertical plane, as a wall.

3. To yield; to give way; to cease opposing. [Obs.]

   The Dutch, as high as they seem, do begin to buckle.
   --Pepys.

4. To enter upon some labor or contest; to join in close fight; to struggle; to contend.

   The bishop was as able and ready to buckle with the Lord Protector as he was with him.
   --Latimer.

   In single combat thou shalt buckle with me.
   --Shak.

   To buckle to, to bend to; to engage with zeal.

   To make our sturdy humor buckle thereto.
   --Barrow.

   Before buckling to my winter's work.
   --J. D. Forbes.