Green’s Dictionary of Slang

1. (US) an only child.

2002

C. Stella Jimmy Bench-Press 14: Larry’s a loner, no brothers or sisters.

2. (N.Z. prison) a prisoner who is either ostracised by their peers or opts for solitude and resists joining any prison groups or gangs.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 110/1: loner n. 1 an inmate who spends his time alone, rather than in a gang or a crew, either through choice or by ostracism (see on the coat).