The Collaborative International Dictionary of English v.0.48:

Tickler \Tic"kler\, n.
   1. One who, or that which, tickles.
      [1913 Webster]

   2. Something puzzling or difficult.
      [1913 Webster]

   3. A book containing a memorandum of notes and debts arranged
      in the order of their maturity. [Com. Cant, U. S.]
      --Bartlett.
      [1913 Webster]

   4. A prong used by coopers to extract bungs from casks.
      [Eng.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tickler
    n 1: a file of memoranda or notices that remind of things to be
         done [syn: tickler, tickler file]