That's how he got a Bowie knife shoved through his belly in a saloon in Virginia City.

5 seconds sound clip from the Back to the Future Part III movie soundboard.

You can hear this line at 01:02:50.150 in the DVD version of the movie.

Quote context

[...]

- Instead, you let him rile you. Rile you into playing his game, his way, by his rules.

- Seamus, relax. I know what I'm doing.

- He reminds me of poor Martin.

- Aye.

- Who?

- Me brother.

- Wait a minute. Wait a minute. You have a brother named Martin McFly?

- Had a brother.

- Martin used to let men provoke him into fighting. He was concerned that people would think him a coward if he refused.

- That's how he got a Bowie knife shoved through his belly in a saloon in Virginia City.

- Never considered the future, poor Martin. God rest his soul.

- Sure, and I hope you're considering the future, Mr. Eastwood.

- I think about it all the time.

- And that crater in the middle north west, the one that's out there all by itself like a starburst... that one's called Copernicus.

[...]