to limber up
Webster's Dictionary (GCIDE)
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]