The Collaborative International Dictionary of English v.0.48:

Pur \Pur\, v. i. [imp. & p. p. Purred; p. pr. & vb. n.
   Purring.] [Of imitative origin; cf. Prov. G. purren.]
   To utter a low, murmuring, continued sound, as a cat does
   when pleased. [Written also purr.]
   [1913 Webster]