to stop in the midst of; to leave undone
「止す」 has another reading with different meanings.
to stop (doing); to cease; to desist; to drop; to lay off; to give up; to quit
to hold up (an umbrella); to extend; to insert; to pour; to apply (color)
雨が降り止した頃に出かけた。
I went out when the rain stopped.
話を途中で止すのは相手に失礼ですよ。
It's rude to stop mid-conversation.
体調が悪くなり彼は仕事を途中で止した。
He stopped his work midway.