The Collaborative International Dictionary of English v.0.48:

Churn \Churn\, v. i.
   To perform the operation of churning.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Churn \Churn\ (ch[^u]rn), n. [OE. chirne, cherne, AS. ceren,
   cyrin; akin to D. karn, Dan. kierne. See Churn, v. t.]
   A vessel in which milk or cream is stirred, beaten, or
   otherwise agitated (as by a plunging or revolving dasher) in
   order to separate the oily globules from the other parts, and
   obtain butter.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Churn \Churn\, v. t. [imp. & p. p. Churned (ch[^u]rnd); p. pr.
   & vb. n. Churning.] [OE. chernen, AS. cernan; akin to LG.
   karnen, G. kernen, D. karnen, Dan. kierne, Sw. k[aum]rna, and
   also to E. corn, kernel, the meaning coming from the idea of
   extracting the kernel or marrow. See Kernel.]
   1. To stir, beat, or agitate, as milk or cream in a churn, in
      order to make butter.
      [1913 Webster]

   2. To shake or agitate with violence.
      [1913 Webster]

            Churned in his teeth, the foamy venom rose.
                                                  --Addison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

churn
    n 1: a vessel in which cream is agitated to separate butterfat
         from buttermilk [syn: churn, butter churn]
    v 1: stir (cream) vigorously in order to make butter
    2: be agitated; "the sea was churning in the storm" [syn:
       churn, boil, moil, roil]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "churn":
   agitate, agitation, agitator, beat, beat up, beater, blender,
   bluster, bobbery, boil, boiling, brouhaha, bubble, bustle,
   cement mixer, churn up, clabber, clot, coagulate, colloid,
   colloidize, commotion, conturbation, convulse, cream, crucible,
   curdle, disarrange, discompose, discomposure, disorder, disquiet,
   disquietude, disturb, disturbance, ebullition, eggbeater,
   embroilment, emulsifier, emulsify, emulsionize, excite, excitement,
   ferment, fermentation, fever, feverishness, fidgets, flap, flurry,
   fluster, flutteration, foam, foment, fret, fume, fuss, hubbub,
   hurly-burly, incrassate, inquietude, inspissate, jell, jellify,
   jelly, jiggler, jitters, jumpiness, lopper, maelstrom, malaise,
   melting pot, mixer, moil, nerviness, nervosity, nervousness,
   paddle, perturb, perturbate, perturbation, restlessness, rile,
   ripple, roil, roughen, rout, row, ruffle, rumple, seethe, seething,
   shake, shake up, shaker, simmer, smolder, stew, stir, stir up,
   swirl, thicken, to-do, trepidation, trepidity, trouble, tumult,
   tumultuation, turbidity, turbulence, turmoil, twitter, unease,
   unrest, upset, vibrator, whip, whip up, whisk, work up