Decided
Settled
noun, の adjective
It was previously decided, which means it's been decided and settled.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
この問題は既決です。
This issue is settled.
それ、もう既決だから、変えられないんだよ。
It's already decided, so we can't change it.
既決の事について話し合う時間はありません。
We don't have time to discuss the decided matters.
既決のルールに従って進めてください。
Please proceed according to the existing rules.