From the Brotherhood 2.0 which officially means Don't Forget To Be Awesome.
However, the Green brothers have charged their nerdfighters to come up with more, equally awesome, meanings for DFTBA. Some examples are
1) Dinosaurs Failed To Beat Astroid
2) Dear, Fetch The Battle Axe
3) Delano Fears To BE Afraid
4) Do Fish Take Baths Alot?
5) Dark Forests Take Bravery Away.
and many, many more. So many that Hank actually wrote a song with only DFTBA acronyms as lyrics, fittingly called "DFTBA."