Source : Webster's Revised Unabridged Dictionary (1913)

Grumbler \Grum"bler\, n.
   One who grumbles.

Source : WordNet®

grumbler
     n : a person given to excessive complaints and crying and
         whining [syn: {whiner}, {complainer}, {moaner}, {sniveller},
          {crybaby}, {bellyacher}, {squawker}]