Because you're a dead shot.
5 seconds sound clip from the The Big Trail (1930) movie soundboard.
You can hear this line at 01:14:36 in the Blu-ray version of the movie.
Quote context
[...]
- I don't mind fists or feet, or even a gun.
- It's the way he throws that knife.
- You're a-going to prove how good you are, before you leave the fort.
[...]