Source : Webster's Revised Unabridged Dictionary (1913)

Kazoo \Ka*zoo"\, n. [Etymol. uncertain.]
   A kind of toy or rude musical instrument, as a tube inside of
   which is a stretched string made to vibrate by singing or
   humming into the tube.

Source : WordNet®

kazoo
     n : a toy wind instrument that has a membrane that makes a sound
         when you hum into the mouthpiece