The Collaborative International Dictionary of English v.0.48:

Sticker \Stick"er\, n.
   1. One who, or that which, sticks; as, a bill sticker.
      [1913 Webster]

   2. That which causes one to stick; that which puzzles or
      poses. [Colloq.] --Tackeray.
      [1913 Webster]

   3. (Mus.) In the organ, a small wooden rod which connects (in
      part) a key and a pallet, so as to communicate motion by
      pushing.
      [1913 Webster]

   4. Same as Paster, 2. [Political Cant, U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

sticker
    n 1: a small sharp-pointed tip resembling a spike on a stem or
         leaf [syn: spine, thorn, prickle, pricker,
         sticker, spikelet]
    2: an adhesive label [syn: gummed label, sticker, paster]
    3: a particularly difficult or baffling question or problem
       [syn: poser, stumper, toughie, sticker]
    4: a short knife with a pointed blade used for piercing or
       stabbing [syn: dagger, sticker]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "sticker":
   Chinese puzzle, adherent, adhesive, baffling problem, barnacle,
   billhead, book stamp, bookplate, brain twister, bramble, brand,
   brier, bristle, broad arrow, bulldog, burr, cachet, cactus,
   catchweed, cement, check, cleavers, colophon, counterfoil,
   countermark, crossword puzzle, crux, decal, decalcomania, docket,
   enigma, enigmatic question, floorer, glue, goose grass,
   government mark, government stamp, gunk, hallmark, imprint,
   jigsaw puzzle, knot, knotty point, label, leech, letterhead,
   limpet, logo, logotype, masthead, mind-boggler, molasses, mucilage,
   mystery, needle, nettle, nut to crack, paste, perplexed question,
   perplexity, pine needle, plaster, plate, poser, price tag, prickle,
   problem, puzzle, puzzlement, puzzler, question, question mark,
   quill, registered trademark, remora, running head, running title,
   seal, sigil, signet, sixty-four dollar question, spicule, spiculum,
   spike, spikelet, spine, stamp, stub, stumper, syrup, tag, tally,
   thistle, thorn, ticket, title page, token, tough proposition,
   trade name, trademark, trademark name, vexed question, why,
   yucca