past events; bygones; water under the bridge
Refers to things that have already happened and are now in the past; often used in expressions about letting go or not dwelling on the past.
What's done is done; there's no use crying over spilled milk.
There's no point in regretting the past forever.
Variant using 事 for こと; slightly more formal or literary feel.
Phrase composed of the past tense of 過ぎる (to pass) + こと (thing/matter), literally 'things that have passed'.