Source : Webster's Revised Unabridged Dictionary (1913)

Sprain \Sprain\, v. t. [imp. & p. p. {Sprained}; p. pr. & vb. n.
   {Spraining}.] [OF. espreindreto press, to force out, F.
   ['e]preindre, fr. L. exprimere. See {Express}, v. t., and cf.
   {Spraints}.]
   To weaken, as a joint, ligament, or muscle, by sudden and
   excessive exertion, as by wrenching; to overstrain, or
   stretch injuriously, but without luxation; as, to sprain
   one's ankle.

Sprain \Sprain\, n.
   The act or result of spraining; lameness caused by spraining;
   as, a bad sprain of the wrist.

   {Sprain fracture} (Med.), the separation of a tendon from its
      point of insertion, with the detachment of a shell of bone
      to which the tendon is attached.

Source : WordNet®

sprain
     n : a painful injury to a joint caused by a sudden wrenching of
         its ligaments
     v : twist suddenly so as to sprain; "wrench one's ankle"; "The
         wrestler twisted his shoulder"; "the hikers sprained
         their ankles when they fell"; "I turned my ankle and
         couldn't walk for several days" [syn: {twist}, {wrench},
         {turn}, {wrick}, {rick}]