used to say that something is only that or ends there; just/only (and nothing more).
彼とはそれっきり会っていない。
I haven't seen him since then.
約束して、それっきり連絡が途絶えた。
We made a promise, and then contact stopped right after.
その話はそれっきりで終わった。
That story ended there (and nothing more happened).
(ever) since; after — often used with a negative to mean 'and from then on (not...)'.
あの日っきり、彼から連絡がない。
Since that day, I haven't heard from him.
卒業してっきり、彼は海外に行ったきり戻ってこない。
After graduating, he went abroad and hasn't come back since.
会議が終わってっきり、彼らは何も報告してこない。
Since the meeting ended, they haven't reported anything.
continuously; remaining in a particular state (used in phrases like '寝っきり' or '黙ったっきり').
ここしばらく寝っきりで家から出ていない。
I've been bedridden for a while and haven't left the house.
彼は黙ったっきり何も話さない。
He stayed silent and didn't say anything.
そのまま放置されっきりでは困る。
It would be a problem if it were just left like that.