Fabian Prewett was a brother of Molly (Prewett) Weasley (JKR, OP9). He died heroically in the first wizarding war.
Fabian and his brother Gideon were among the "best witches and wizards of the age" who were killed during theVoldemort years (PS4). They were part of the original Order of the Phoenix. It took five Death Eaters, including Antonin Dolohov, to kill Fabian and Gideon (OP25). Moody said they died like heroes (OP9).
Molly Weasley gave Harry Potter Fabian's watch on his seventeenth birthday. Fabian was not too carefull with his possessions, so it was battered (DH7).
Part of the Prewett family, brother to Gideon and Molly Weasley-Prewett.
Two of the best wizards of their age, according to Mad-eye Moody (OP9).
Considering that his brother Gideon is named after an underdog military leader of the ancient world, Fabian's given name probably comes from the Roman dictator Quintus Fabius Maximus Verrucosus and his Fabian strategy, a tactic of attrition warfare that he used successfully against Hannibal and his army (source: Wikipedia).
It may or may not be a coincidence that Fred and George share the same initials as Fabian and Gideon.
Gideon and his brother Fabian both seem to be named for ancient military leaders who led underdog armies to victory (sources: Book of Judges, Wikipedia). -BB