to limber up

from 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]