I believe dfa in relation to Destiny 2 is the Tree of Probabilities Nightfall reward hand cannon, with DFA's initials meaning Don't F'n Ask or Didn't Find Anything I still don't know myself. And I had to look up KTWD, which I found out you have mixed up with KWTD, which is Know What To Do.
English
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Dfa is for death from above or don't -blam!- around
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You're the first person I've seen use Death From Above for the D.F.A. hand cannon, and it makes a lot more sense than what I've said. Don't F Around also makes a lot more sense.