Green’s Dictionary of Slang

(US prison) a disciplinary report.

1950		Goldin et al. DAUL 240/1: Write-up, n. (P) A formal report lodged against an inmate for rule violation.
1992		Bentley & Corbett Prison Sl. 103: Write-Up When an inmate is found in violation of prison rules, he is given a ticket or write-up. This is a standard disciplinary form stating the offense, time and day the infraction occurred and disposition procedures.
2002		J. Lerner You Got Nothing Coming 206: Ringer is ripping up another of Stanger’s write-ups.