Bygone means belonging to a time now long past. A bygone age, bygone days, bygone customs.
It is nostalgic and almost always attached to age, era or days. Let bygones be bygones is the fixed phrase, meaning let past quarrels go — and there the plural noun means the past offences themselves. Note that the word is simply gone by, turned round.
1adjectivebygoneBelonging to a time now long past.
“The old photographs show a bygone age of horse-drawn carts and gas lamps.”
bygone is a quality of pastness.
From by + gone. See go by.
Etymology from Wiktionary, CC BY-SA