five-eight, n. — Green’s Dictionary of Slang

five-eight n.

[rhy. sl. = mate n.¹ (1); ult. a position in the game of rugby]

(Aus. prison) a friend.

1968		J. Alard He who Shoots Last 136: If he’s yer five-eight ya got nuttin’ ta be noivous about.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Five-eight. Friend.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 70/1: five-eight n. one’s best mate.