The Collaborative International Dictionary of English v.0.48:

Birr \Birr\, v. i. [imp. & p. p. Birred; p. pr. & vb. n.
   Birring.] [Cf. OE. bur, bir, wind, storm wind, fr. Icel.
   byrr wind. Perh. imitative.]
   To make, or move with, a whirring noise, as of wheels in
   motion.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Birr \Birr\, n.
   1. A whirring sound, as of a spinning wheel.
      [1913 Webster]

   2. A rush or impetus; force.
      [1913 Webster]

WordNet (r) 3.0 (2006):

birr
    n 1: the basic unit of money in Ethiopia; equal to 100 cents
    2: sound of something in rapid motion; "whir of a bird's wings";
       "the whir of the propellers" [syn: whir, whirr,
       whirring, birr]
    v 1: make a soft swishing sound; "the motor whirred"; "the car
         engine purred" [syn: whizz, whiz, whirr, whir,
         birr, purr]