Why does Brienne name her sword Oathkeeper?

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Tags

1. #charging
2. #tywin
3. #presents
4. #whereabouts
5. #oathkeeper
6. #tarth
7. #lannister
8. #sansa
9. #jaime
10. #instead
11. #gives
12. #asking
13. #finding
14. #brienne
15. #sword
16. #elder
17. #stark