Oh bootie bootie
Oh yea
Oh baby bootie
Yea yea yea

The story:

I was sitting in my friend's room, and the radio was on. Baby One More Time started playing, and we were surprised that the station was still playing it, being that it is so old.
My friend and I decided to sing along, and I belted out the lines in quesiton, and she fell on the floor laughing.
It took me a few minutes to figure out why, and when I finally did, I was really embarassed, because through all of these years, I never knew the right words.