I don't know, Doc.
3 seconds sound clip from the Back to the Future movie soundboard.
You can hear this line at 00:56:41 in the Blu-ray version of the movie.
Quote context
[...]
- Jesus, didn't that guy ever have hair?
- Shape up, man. You're a slacker. Do you want to be a slacker for the rest of your life?
- No.
- What did your mother ever see in that kid?
- I guess she felt sorry for him 'cause her dad hit him with the car.
[...]