The Collaborative International Dictionary of English v.0.48:

Sprain \Sprain\ (spr[=a]n), v. t. [imp. & p. p. Sprained
   (spr[=a]nd); 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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sprain \Sprain\, n.
   The act or result of spraining; lameness caused by spraining;
   as, a bad sprain of the wrist.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sprain
    n 1: a painful injury to a joint caused by a sudden wrenching of
         its ligaments
    v 1: 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, sprain, wrench,
         turn, wrick, rick]