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]