Indicates that something ended at that point and nothing followed afterward (i.e., 'and that was it').
彼は手紙を一通残して、それきり戻ってこなかった。
He left one letter and that was it — he never returned.
電話でそう言われてそれきり話が途絶えた。
After being told that on the phone, the conversation stopped there.
彼女とはその日会って以来、それきり連絡を取っていない。
I met her that day and haven't had any contact with her since.