Green’s Dictionary of Slang

flat adv.

(US prison) without remission.

1984

A. Sample (con. late 1950s-early 1960s) Racehoss 162: Big Devil decided he should do his two year sentence ‘flat.’ This meant instead of getting out in the normal fourteen months and twelve days, the equivalent of two years with good time, he would serve the full two calendar years.

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