The Collaborative International Dictionary of English v.0.48:

kazoo \ka*zoo"\, n. [Etymol. uncertain.]
   A kind of toy or rude musical instrument, as a tube inside of
   which is a stretched string or membrane made to vibrate by
   singing or humming into the tube.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Mirliton \Mir"li*ton\, n. [F.]
   1. A kind of musical toy into which one sings, hums, or
      speaks, producing a coarse, reedy sound. It is also called
      a kazoo.
      [Webster 1913 Suppl. +PJC]

            Trilby singing "Ben Bolt" into a mirliton was a
            thing to be remembered, whether one would or no!
                                                  --Du Maurier.
      [Webster 1913 Suppl.]

   2. same as chayote[2].
      [PJC]

WordNet (r) 3.0 (2006):

kazoo
    n 1: a toy wind instrument that has a membrane that makes a
         sound when you hum into the mouthpiece