Green’s Dictionary of Slang

(US prison) one’s best clothes.

1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 198: bonnaroos, n. – well-pressed clothes and shined shoes.
1976		M. Braly False Starts 235: Is he a gunsel boneroos and a reputation to protect?
1990		S. Morgan Homeboy 191: He wore his best starched and pressed prison blues, his bonaroos, and a blue watchcap.
2000		Other Side of the Wall: Prisoner’s Dict. July 🌐 Bonaroo: One’s best clothes. ‘I’ve got my bonaroos all ready for my next visit.’.
2001		(con. 1975–6) E. Little Steel Toes 9: You have got to have your bonaroos, pressed khakis, clean T-shirt, and shoes or boots shined till they look like black mirrors.