Sometimes called Tome of Battle: Nine Euphemisms for My Dick, The Book of Weeaboo Fightan Magic is the nickname given to The Tome of Battle: The Book of Nine Swords, one of the more famous (or infamous) Dungeons & Dragons 3rd Edition (3.5) splatbooks. It introduced three new classes, Crusader, Swordsage and Warblade, and a wide variety of abilities referred to as Stances and Maneuvers, which basically function as spell-substitutes for martial characters. This was supposed to lessen the power gap between martial and magical characters.
By rendering all the existing fighting classes irrelevant. Oh no wait, it was the spellcasting classes that did that. Fuck.
Opinion is divided on the quality of the content of the book itself. Some neckbeards believe the Stances and Maneuvers are interesting and worthy additions to the game, whereas others consider them to be bizarre and illogical, comparing them to some of the ridiculous techniques seen in Japanese anime and manga. This comparison is what resulted in the book being assigned its nickname, which has indeed become so pervasive that even some neckbeards who like the content call it Weeaboo Fightan Magic.
Everyone agrees, however, that it makes people who only play bathrobe faggots extremely angry.
Edit: Also known as the book of the nine beards, causing all those in the know to scream "CoooooooS" at the top of their lungs.