Green’s Dictionary of Slang

(Aus. prison) good quality (non-jail) tobacco.

1902

‘The Wasp’ in ‘Tales of the Penance Track’ in Bulletin (Sydney) 31 May 31/1: MacManus was discontented, and The Spider industriously fanned the discontent, thereby producing many small chews of ‘flat.’ The Spider was a glutton for ‘weed’; the gaol tobacco had lost him any small chance of salvation he might have possessed.