I think it was left at "Jack I swear" to show the limitless possibilities.
There is so much that could follw that line that I don't think Ennis even know how to finish it.
I don't know about in the west, but here in the south we say that a lot.
"Did you hear Myrtle Jean down ta the church fell yesterdy?" No! Well I swear.
Or if someone pisses you off ( Iuse it on my daughter a lot) Girl, I swear!
It's an all around phrase that kinda says nothing but gets the point across.
Anyway, I think theres just too much going on in Ennis' head and heart that thats all he could get out.
But, thats just me.