One, especially a grievance, that is past: Let bygones be bygones. (noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "bygone" in a sentence
"The words used in bygone eras were not as sharp or short as they are now, and neither were the phrases."
"Today this is a hill near San Juanito Escobedo — locally known as Las Cuevas — but in bygone days this hill was actually an island in the middle of now-drained Lake Magdalena."
"I lived near to what was a major public order situation in bygone years, and found interestingly that despite the fact I had to commute through it several days running, not once was i harrassed, stopped or bothered by the police."