You were standing on your toilet and you were hanging a clock, and you fell and you hit your head on the sink.
7 seconds sound clip from the Back to the Future movie soundboard.
You can hear this line at 00:50:42 in the Blu-ray version of the movie.
Quote context
[...]
- Doc, you gotta listen to me.
- I've had enough practical jokes for one evening.
- The bruise-the bruise on your head, I know how that happened. You told me the whole story.
- There's something wrong with the starter, so I hid it here.
- After I fell off my toilet, I drew this.
[...]