Tickler \Tic"kler\, n.

1. One who, or that which, tickles.

2. Something puzzling or difficult.

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

--Bartlett.

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

Source: Webster's Revised Unabridged Dictionary (1913)