既往 is read as きおう (kiou) and means the past.
きおう
kiou
きおうしょう
illness that one had in the past
きおうれき
case history
きおうはとがめず
don't dwell on the past
きせい
established
きそん
existing
すでに
already
すんでに
almost
ready-made
きせいふく
ready-made clothes
きてい
きせいじじつ
established fact
きとくけん
vested rights