Green’s Dictionary of Slang

(orig. US black, also dead solid) trustworthily, dependably, excitingly, outstandingly.

1921		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.
1930		D. Hammett ‘The First Thin Man’ in Nightmare Town (2001) 376: He’s in solid with the police.
1937		F.C. Painton ‘The Devil Must Pay’ in Goodstone Pulps (1970) 19/2: This will put us in solid with the men.
1949		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’?
1955		B. Schulberg On the Waterfront (1964) 207: When all the smoke and bullshit blew away we was still in solid.
1959		Murtagh & Harris Who Live In Shadow (1960) 52: My songs came zooming out of me down-home solid. I was sure of myself.
1960		‘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.
1974		D. Jenkins Dead Solid Perfect 146: And he's dead solid getting it on with a girl who's got her back turned to the camera.
1978		R. Daley Prince of the City 114: [Agent] Bario is in solid with Perrazzo.
2004		J. Ellroy ‘Hot-Prowl Rape-O’ in Destination: Morgue! (2004) 265: Come on. Danny did us all solid.