The didn't-do list is list of things you intended to do at the beginning ofa month or a week, or even a day, but failed to get them done by the planneddeadline. It is a to-do list that keeps a record of undone things.

“未办事项列表”指在每个月、每周甚至每天开始的时候，列出一些打算完成的事项，可在计划的截止时间却发现事项都没有完成。“未办事项列表”其实就是没有按计划完成的待办事项列表。托福词汇下载

这个didn't-do list其实就是在to-do list的基础上衍生出来的，to do表示计划要做，而过去式的didn'tdo则表示没有完成。

For example:

Every morning, I add some tasks to my to-do list, and by the time I leavemy office in the afternoon, there are so many undone things on the list that ithas grown into a didn't-do list.

每天早晨，我都会往待办事项列表里添加一些要完成的任务，可是到了下午下班的时候，列表中的未完成事项还是很多，以至于待办事项列表都变成未办事项列表了。