Green’s Dictionary of Slang

smartly, neatly, esp. of dress.

1971		Frank Zappa ‘Shove It Right In’ 🎵 Somewhat desirable boys there / Dressed really spiffy, with long hair.
1988		D. Woodrell Muscle for the Wing 110: I could dress spiffy and talk like a duke, too.
1997		H.S. Thompson Proud Highway xxii: Always Dress Spiffy.