Wanna hear a really nasty story?? Urine luck.

IndyCar star Conor Daly admitted to wetting himself as he waited out the start of the Indianapolis 500 on Sunday ... revealing he had to do the entire race "sitting in my own pee."

The 33-year-old Indiana native opened up about his race experience at Monday's victory banquet ... and while he finished 8th in the event, the takeaway was more about No. 1 -- specifically during the rain delay that postponed the start by about 45 minutes.

"Never in my life have I urinated in my car until Sunday," Daly said as the room exploded in laughter. "I was sitting on the grid, and I was like, 'This is the best car I've ever been in in my whole life. I'm gonna have to pee in this thing.' I kid you not."

"I legitimately urinated in my racecar before the race even started."

When asked if it was due to the excitement or his bladder filling up, Daly said he "had to go really bad."

Of course, there was no other option for Daly ... as he had to sit in his car and wait for the weather to cooperate.

He was able to crack some jokes about it all ... saying he wasn't sure if it was his diabetes acting up or just old age -- something Helio Castroneves would know about.

"He's halfway to 100 [years old]," Daly said lightheartedly. "I don't know if that happens to him a lot."