The Collaborative International Dictionary of English v.0.48:

Curdle \Cur"dle\ (k[^u]r"d'l), v. i. [From Curd.] [Sometimes
   written crudle and cruddle.]
   1. To change into curd; to coagulate; as, rennet causes milk
      to curdle. --Thomson.
      [1913 Webster]

   2. To thicken; to congeal.
      [1913 Webster]

            Then Mary could feel her heart's blood curdle cold.
                                                  --Southey.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Curdle \Cur"dle\, v. t. [imp. & p. p. Curdled (-d'ld); p. pr.
   & vb. n. Curdling (-dl[i^]ng).]
   1. To change into curd; to cause to coagulate. "To curdle
      whites of eggs" --Boyle.
      [1913 Webster]

   2. To congeal or thicken.
      [1913 Webster]

            My chill blood is curdled in my veins. --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

curdle
    v 1: turn into curds; "curdled milk" [syn: curdle, clabber,
         clot] [ant: homogenise, homogenize]
    2: go bad or sour; "The milk curdled"
    3: turn from a liquid to a solid mass; "his blood curdled"