n. 难题, 难事, 备忘录, 呵痒的人

[经] 备忘录, 记事本, 到期票据记录簿

[ noun ]
a file of memoranda or notices that remind of things to be done
<noun.communication>

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.]