bygone
1. [ adjective ] well in the past; former
Synonyms: gone departed foregone bypast
Examples:

"bygone days" "dreams of foregone times" "sweet memories of gone summers" "relics of a departed era"

Related terms: past
2. [ noun ] past events to be put aside
Synonyms: water_under_the_bridge
Examples:

"let bygones be bygones"

Related terms: past