Green’s Dictionary of Slang

1. (US) one who moves fast, either physically or mentally; also in ironic use.

1953		R. Chandler Long Good-Bye 40: He’s a swifty. He moves fast. Good reflexes.
1967–8		Baker et al. CUSS 207: Swifty A quick or wittty person.
1989		C. Hiaasen Skin Tight 66: Boy, are you a swifty.

2. a quick blow.

c.1973

J. Thompson ‘Sunrise at Midnight’ in Fireworks (1988) 173: I got him with two swifties, a quick one-two with my knees.

3. see swift one under swift adj.