Question: What did Ennis mean at the end of the movie when he says "Jack, I swear"? It seemed really out of place.
Answer: There's been a lot of discussion as to what this line actually means, but there's no definitive answer.
I feel like it is a statement of loss, what they had, what could have been, but Jack is now gone. They will never be together again, but the strong feelings of love and hurt that Ennis has been, and always will be, there.
Answer: I feel it meant "I swear our love will never die." Or "I swear I'll love you forever."