A special time, typically from around 9pm to midnight when you and your group of buddies start to laugh uncontrollably at the most stupid of jokes, obscure references and even ambient noises happening around you.
This mostly occurs when hanging out in a stressful or tiring environment (work, convention, birthday etc)
Typically, retard o'clock will get to a climax when ultimately, something will cause everyone to cry with laughter - equally as dumb as everything else uttered - for a few minutes straight, with some people possibly vacating company in order to isolate the chained laughter.The reason for what initially caused you all to laugh in the first place will be immediately forgotten.
I can't even believe that could be that funny, I had to run out of the room crying... and I don't even remember why. Must be Retard O'clock. It's way too late.