A: 百均に入ると、何もいらなくても何か買わないと気が済まない人っているんだよね。
B: いるいる。僕だよ。
A: At the dollar store, there is always somebody who has to buy something even if they don't need it.
B: Yes, yes! That's me.
I am told that 気が済まない means "won't be satisfied unless," or when one "must (do something)."
If I translate the expression literally I get confused--
気 spirit/mood/feeling
が
済まない finish; come to an end; excusable; need not
Would anyone be kind enough to give me a clear and thorough explanation of this expression? I think my main confusion is as to which verb 済まない comes from.