The Collaborative International Dictionary of English v.0.48:

bummer \bum"mer\ (b[u^]m"[~e]r), n.
   1. An idle, worthless fellow, who is without any visible
      means of support; a loafer; a dissipated sponger; one who
      bums. [Slang, U.S.]
      [1913 Webster]

   2. an unpleasant event, experience, or situation; as, getting
      caught in a cloudburst while wearing my best suit was a
      bummer. [Slang, U.S.]
      [PJC]

WordNet (r) 3.0 (2006):

bummer
    n 1: an experience that is irritating or frustrating or
         disappointing; "having to stand in line so long was a real
         bummer"
    2: a bad reaction to a hallucinogenic drug

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "bummer":
   Arab, Bowery bum, adverse circumstances, adversity, affliction,
   aggravation, annoyance, bandit, beach bum, beachcomber, beggar,
   beggarly fellow, blight, blighter, bo, brigand, budmash, bum,
   cadger, caitiff, care, coupon clippers, cross, curse, depredator,
   derelict, devil, difficulties, difficulty, dogie, downer, drifter,
   drone, drunkard, forager, freebooter, freeloader, gamin, gamine,
   good-for-naught, good-for-nothing, guttersnipe, hard knocks,
   hard life, hard lot, hardcase, hardship, hobo, homeless waif,
   human wreck, idle rich, idler, irritation, landloper, lazzarone,
   leisure class, loafer, looter, losel, lounge lizard, lowlife,
   lumpen proletariat, mauvais sujet, mean wretch, mendicant,
   mendicant friar, mendicant order, moocher, mucker, mudlark,
   no-good, nonworker, panhandler, parasite, pauvre diable, piker,
   pilgarlic, pillager, plight, plunderer, poor creature, poor devil,
   predicament, pressure, ragamuffin, ragman, ragpicker, rentiers,
   rigor, rounder, sad case, sad sack, schnorrer, scrounger,
   sea of troubles, ski bum, skid-row bum, spiv, sponger, stiff,
   stray, street Arab, street urchin, stress, stress of life,
   sundowner, surf bum, swagman, swagsman, tatterdemalion, tennis bum,
   the unemployable, the unemployed, tramp, trial, tribulation,
   trouble, troubles, truant, turnpiker, urchin, vag, vagabond,
   vagrant, vale of tears, vaurien, vicissitude, waif,
   waifs and strays, wastrel, worthless fellow, wretch