Green’s Dictionary of Slang

(N.Z. prison) using a body-building technique, for a prisoner, on the verge of a fight, to hold himself in such a way that makes him look bigger and tougher than he actually is.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 14/1: have basketballs under one’s arms describes the way an inmate looks when he strikes a defensive posture of drawing himself up to his full height, rolling his back forward to extend it and simultaneously pushing his shoulders and chest up in an attempt to look bigger and tougher than he is. The basis of this posture is a bodybuilding pose known as a ‘front lat spread’.