Green’s Dictionary of Slang

scrap iron n.

1. as an alcoholic drink.

(a) (US) homemade whisky, bad liquor.

[US]Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1002: Jelly figured that if he bullskated just right, he might confidence Sweet Back out of [...] a shot of scrap-iron or a reefer.
[US]M.H. Boulware Jive and Sl.

(b) (UK prison) a homemade alcoholic drink; see cit. 1992.

[US]Bentley & Corbett Prison Sl. 71: Scrap Iron A drink made with rubbing alcohol, mothballs and a chlorine solution which has intoxicating properties.

2. (US prison) weights, used for exercising and body-building.

[US]Maledicta V:1+2 Summer + Winter 267: Other terms commonly heard in prison life are [...] scrap iron for weights for exercising.