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

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "bumper":
   aegis, arm guard, backstop, barrier, bellyful, beverage, big,
   brattice, buffer, buffer state, bulkhead, bulwark, capacity,
   charge, cloison, collision mat, complement, considerable,
   contraceptive, copyright, cram, crash helmet, crush, cushion,
   dashboard, diaphragm, dissepiment, dividing line, dividing wall,
   division, dodger, draft, dram, drench, drink, face mask, fender,
   fill, finger guard, foot guard, full house, full measure, fuse,
   goggles, goodly, governor, grand, great, guard, guardrail, gulp,
   guzzle, hand guard, handrail, hard hat, healthy, helmet,
   insulation, interlock, interseptum, jam up, knee guard,
   knuckle guard, lading, laminated glass, lap, large, large-scale,
   libation, life preserver, lifeline, lightning conductor,
   lightning rod, load, man-sized, mask, mat, midriff, midsection,
   mouthful, mudguard, nip, nose guard, numerous, pad, padding,
   palladium, panel, paries, partition, party wall, patent, peg,
   pilot, portion, potation, preventive, property line, prophylactic,
   protective clothing, protective umbrella, pull, quaff, safeguard,
   safety, safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, screen, seat belt, separation,
   septulum, septum, shield, shin guard, shock pad, sip, sizable,
   skinful, slurp, snootful, snort, substantial, suck, sun helmet,
   sup, swig, swill, tall, tidy, tot, umbrella, wall, windscreen,
   windshield