The Collaborative International Dictionary of English v.0.48:

Sewellel \Se*wel"lel\, n. [Of American Indian origin.] (Zool.)
   A peculiar gregarious burrowing rodent (Haplodon rufus),
   native of the coast region of the Northwestern United States.
   It somewhat resembles a muskrat or marmot, but has only a
   rudimentary tail. Its head is broad, its eyes are small and
   its fur is brownish above, gray beneath. It constitutes the
   family Haplodontidae. Called also boomer, showt'l, and
   mountain beaver.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Boomer \Boom"er\, n.
   1. One who, or that which, booms.
      [1913 Webster]

   2. (Zool.) A North American rodent, so named because it is
      said to make a booming noise. See Sewellel.
      [1913 Webster]

   3. (Zool.) A large male kangaroo.
      [1913 Webster]

   4. One who works up a "boom". [Slang, U. S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

boomer
    n 1: a member of the baby boom generation in the 1950s; "they
         expanded the schools for a generation of baby boomers"
         [syn: baby boomer, boomer]