noun
unfinished work; thing left undone
Refers to tasks or work that one has not completed, often with a nuance of regret or something that needs to be finished later.
し残しの仕事がまだたくさんある。
There is still a lot of unfinished work.
今日のし残しを明日に持ち越さないようにしよう。
Let's not carry over today's unfinished tasks to tomorrow.
やり残し is more common and general for things left undone, while し残し is a less frequent synonym with a similar meaning.
Compound of し (continuative form of する 'to do') + 残し (continuative form of 残す 'to leave behind').