Green’s Dictionary of Slang

solid adv.1

[solid adj.]

(orig. US black) trustworthily, dependably, excitingly, outstandingly.

[US]O.O. McIntyre Bits of New York Life 6 Feb. [synd. col.] Folk who like friends to believe they stand solid with theatrical producers buy their tickets and then buy a puncher to complete the bit of four flushing.
[US]D. Hammett ‘The First Thin Man’ in Nightmare Town (2001) 376: He’s in solid with the police.
[US]F.C. Painton ‘The Devil Must Pay’ in Goodstone Pulps (1970) 19/2: This will put us in solid with the men.
[US]I. Shulman Cry Tough! 81: Remember [...] when we were tryin’ to get in solid with some of the bigger guys so when they’d ask us to work someone over, we useta do it for nothin’?
[US]B. Schulberg On the Waterfront (1964) 207: When all the smoke and bullshit blew away we was still in solid.
[US]Murtagh & Harris Who Live In Shadow (1960) 52: My songs came zooming out of me down-home solid. I was sure of myself.
[US]‘Lord Buckley’ Hiparama of the Classics 15: Solid sent upon the Ace Lick that all Cats and Kitties, Red, White, or Blue are created level, in front.
[US]J. Ellroy ‘Hot-Prowl Rape-O’ in Destination: Morgue! (2004) 265: Come on. Danny did us all solid.

In phrases

stick solid (v.)

(Aus. Und.) to remain silent under interrogation.

[Aus]M.B. ‘Chopper’ Read How to Shoot Friends 174: At least the kid stuck solid, and [...] he didn’t tell the teachers how his head was opened up.