1. 
2. 
[syn: fastball, heater, smoke, hummer, bullet]
The Collaborative International Dictionary of English v.0.48:
Hummer \Hum"mer\, n.
   1. One who, or that which, hums; one who applauds by humming.
      --Ainsworth.
      [1913 Webster]
   2. (Zool.) A humming bird.
      [1913 Webster]
WordNet (r) 3.0 (2006):
hummer
    n 1: a singer who produces a tune without opening the lips or
         forming words
    2: (baseball) a pitch thrown with maximum velocity; "he swung
       late on the fastball"; "he showed batters nothing but smoke"
       [syn: fastball, heater, smoke, hummer, bullet]