Green’s Dictionary of Slang

1. (US campus) successfully.

1834		‘Girl of the Town’ in Bang-Up Songster 24: Then vot Countess or duchess of wealth and renown, / Comes it half so fat as a girl of the town?
1889		H. Lawson ‘Sleeping Beauty’ in Roderick (1967–9) I 60: ’Tis very plain he’d struck it fat, / This dufferin’ London muff.
1902		P.L. Dunbar Jest Of Fate (1903) 167: When, as she would have expressed it, ‘everything was going fat,’ she suddenly paused.
1997–2000		Da Bomb 🌐 5: Busting fat: Doing something very well.

2. comfortably.

1903		J. London People of the Abyss 92: ‘There’s mugs never go out of Kent,’ spoke a second voice, ‘they live bloomin’ fat all along.’.
1911		C.E. Mulford Bar-20 Days 136: There he flourished and waxed fat.
1952		W. Sheldon Troubling of a Star 35: You guys in the Air Corps really have it fat.

3. very, extremely.

1972

B. Rodgers Queens’ Vernacular 79: fat righteous (adv) 1. a lot ‘I dig you fat righteous’ 2. really, truly.