The Collaborative International Dictionary of English v.0.48:

Bumper \Bum"per\, n. [A corruption of bumbard, bombard, a large
   drinking vessel.]
   1. A cup or glass filled to the brim, or till the liquor runs
      over, particularly in drinking a health or toast.
      [1913 Webster]

            He frothed his bumpers to the brim.   --Tennyson.
      [1913 Webster]

   2. A covered house at a theater, etc., in honor of some
      favorite performer. [Cant]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bumper \Bump"er\, n.
   1. That which bumps or causes a bump.
      [1913 Webster]

   2. Anything which resists or deadens a bump or shock, such as
      a metal or rubber rim extending from an object; a buffer.
      [1913 Webster]

   3. Specifically: (Motor vehicles) A protective guard device,
      usually of metal or rubber, attached horizontally to the
      front or rear of the frame of a vehicle, designed to
      resist or deaden a bump or shock, and to prevent damage to
      the main frame of the vehicle in low-velocity collisions.
      [PJC]

WordNet (r) 3.0 (2006):

bumper
    n 1: a glass filled to the brim (especially as a toast); "we
         quaffed a bumper of ale"
    2: a mechanical device consisting of bars at either end of a
       vehicle to absorb shock and prevent serious damage