Green’s Dictionary of Slang

(US teen) terrific! splendid! wonderful!

1962		Ragen & Finston World’s Toughest Prison 809: nifty – Desirable; excellent.
1972		Dahlskog Dict. Contemp. and Colloq. Usage 42/1: Niftykeen, a. Great; wonderful; admirable.
1982		R.M. Brown Southern Discomfort (1983) 66: ‘Nifty beans,’ Athena said, ‘No slang in this house, miss.’ Ada frowned.