The hour of ridiculous, so late at night or early in the morning that it can't be expressed by a concrete hour.
Though it generally refers to the time between 3:00 AM and 7:00 AM, it can be used in place of any time the speaker finds absurd.
"I can't go home with her, I have work in the morning and she stays out till like, ridic-O'clock.
A time of morning (or night), usually between 3AM - 6AM in the local time zone that requires a person to interrupt normal sleep or waking patterns for often dubious reasons. Often used in reference to extremely early arrivals to airports, or to join business teleconference sessions with participants in multiple time zones. Also see stupid o'clock.
"I don't look so good because I had to be at work at silly o'clock this morning to join a videocon with our development office in Greece."