Source : Webster's Revised Unabridged Dictionary (1913)

Rack \Rack\, v. i. [imp. & p. p. {Racked}; p. pr. & vb. n.
   {Racking}.] [See {Rack} that which stretches, or {Rock}, v.]
   To amble fast, causing a rocking or swaying motion of the
   body; to pace; -- said of a horse. --Fuller.

Racking \Rack"ing\, n. (Naut.)
   Spun yarn used in racking ropes.

Source : WordNet®

racking
     adj : causing great physical or mental suffering; "a wrenching
           pain" [syn: {wrenching}]