Green’s Dictionary of Slang

do-pop n.

[ety. unknown]

(US prison) a semi-trusty.

[US]B. Jackson Killing Time 174: Frank got turned out on a highpower over the plow squad. I had got turned out do-pop [a half trusty] and was working at the horse barn. [Ibid.] 179: He had been busted from do-pop to a rank man.