I've always roleplayed Bardic Knowledge coming from connections made to mythology, legends, sagas, etc.
Which leads me to my favorite house rule, allowing bardic knowledge to know about monsters and artifacts. The DC for monsters is 40 - CR, thus the greater the monster the more legendary knowledge about it. For magic items it is 30 - 1 for every 10,000 gp of market price. For artifacts it is a flat DC of 20. A successful roll gives you knowledge about the monsters capabilities, weaknesses, or things like command words.
When I write adventures, I add a Bardic knowledge table with references like "In the Saga of Gorgonard, he carries a staff that keeps him from ever tiring or feeling fatigue. It is said he laid gave it to a young blue dragon in exchange for his life."
Makes life cool.