The Collaborative International Dictionary of English v.0.48:

Limber \Lim"ber\ v. t. [imp. & p. p. Limbered
   (l[i^]m"b[~e]rd); p. pr. & vb. n. Limbering.] (Mil.)
   To attach to the limber; as, to limber a gun.
   [1913 Webster]

   To limber up, to change a gun carriage into a four-wheeled
      vehicle by attaching the limber.
      [1913 Webster]