Source : Webster's Revised Unabridged Dictionary (1913)

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}.]