Source : Webster's Revised Unabridged Dictionary (1913)

Bolt \Bolt\, adv.
   In the manner of a bolt; suddenly; straight; unbendingly.

         [He] came bolt up against the heavy dragoon.
                                                  --Thackeray.

   {Bolt upright}.
   (a) Perfectly upright; perpendicular; straight up;
       unbendingly erect. --Addison.
   (b) On the back at full length. [Obs.] --Chaucer.