Source : Webster's Revised Unabridged Dictionary (1913)

Brownie \Brown"ie\, n. [So called from its supposed tawny or
   swarthy color.]
   An imaginary good-natured spirit, who was supposed often to
   perform important services around the house by night, such as
   thrashing, churning, sweeping. [Scot.]

Source : WordNet®

brownie
     n 1: (folklore) fairies that are somewhat mischievous [syn: {elf},
           {hob}, {gremlin}, {pixie}, {pixy}, {imp}]
     2: square or bar of very rich chocolate cake usually with nuts