From 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]