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]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "sprain":
   abrade, bark, blemish, bloody, break, burn, chafe, check, chip,
   claw, crack, craze, cut, dislocate, fracture, fray, frazzle, fret,
   gall, gash, hurt, incise, injure, lacerate, maim,
   make mincemeat of, maul, mutilate, pierce, pull, puncture, rend,
   rick, rip, run, rupture, savage, scald, scorch, scotch, scrape,
   scratch, scuff, skin, slash, slit, stab, stick, strain, stretch,
   tear, throw, traumatize, turn, twist, wound, wrench