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]

The Collaborative International Dictionary of English v.0.48:

Pur \Pur\, n.
   The low, murmuring sound made by a cat to express contentment
   or pleasure. [Written also purr.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Purr \Purr\, n.
   The low murmuring sound made by a cat; pur. See Pur.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Purr \Purr\, v. i. & t.
   To murmur as a cat. See Pur.
   [1913 Webster]

WordNet (r) 3.0 (2006):

purr
    n 1: a low vibrating sound typical of a contented cat
    v 1: make a soft swishing sound; "the motor whirred"; "the car
         engine purred" [syn: whizz, whiz, whirr, whir,
         birr, purr]
    2: indicate pleasure by purring; characteristic of cats [syn:
       purr, make vibrant sounds]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "purr":
   be in heaven, be pleased, beam, birr, bombilate, bombinate, boom,
   bum, burr, buzz, delight, die with delight, drone, feel happy,
   go into raptures, hum, joy, laugh, sing, smile,
   take great satisfaction, thrum, tread on air, whir, whiz