bygone
Concise Oxford English Dictionary © 2008 Oxford University Press:
bygone/ˈbʌɪɡɒn/
▶adjective belonging to an earlier time.
– phrases
let bygones be bygones forget past differences or offences and be reconciled.
let bygones be bygones forget past differences or offences and be reconciled.
'bygone' also found in these Oxford entries:

