Green’s Dictionary of Slang

down adv.3

[abbr. go down the river under river n. or down the river under river n.]

(US Und.) serving time in prison.

[Aus]G.C. Mundy Our Antipodes I 111: [of transportation] That implicit trust which those who have never ‘been down’ have a right to respect.
[US]‘Boxcar Bertha’ Sister of the Road (1975) 214: My old pimp [...] got pinched for carrying some junk in to a prisoner, ands he’s been down at Atlanta for five years.
[US]C. Himes Big Gold Dream 104: I’ve been down so long that down don’t worry me.
[US]E. Bunker Animal Factory 12: You’ll look like Gina Lollobrigida to some of those animals who’ve been down for eight or nine years.
[UK]J. Sullivan ‘To Hull and Back’ Only Fools and Horses [TV script] Oh I’ll have him down.
[US]Simon & Burns Corner (1998) 147: I mean, she stood by me when I was down.
[US]J. Lerner You Got Nothing Coming 13: What’s up, dawg? Whatchu down for this time?
[US]J. Stahl Happy Mutant Baby Pills 28: Son, I was down while you were still boosting Slurpees from the 7-Eleven.