Green’s Dictionary of Slang

1. a streetwise young person.

1987		L. Redmond Emerald Square 309: ‘Spacers’ they call them now, but I preferred one spacer to a dozen of these college types.
1988		R. Doyle Commitments 10: We’ll leave the reggae to the skinheads an’ the spacers.

2. a crazy person.

1998		J. O’Connor Salesman 203: Leave it, y’fuckin’ spacer. You’ll kill the cunt.
2005		P. Howard Curious Incident of the Dog in the Nightdress 144: He was just stood there [...] staring at it like a total spacer.