It's a term for when we do bad choices here in life. Really really bad choices. The very worst of choices to be exact. And always something we will end up regretting for the rest of our lives. The very foul ups that will haunt us to the day we die and stay there as an ever lasting reminder of what was done.
Joe: Oh man, i did a biggie, i got thrown out by my parents because i wrecked their car.
Jane: Oh wow, yeah you REALLY did a biggie there Joe.