Green’s Dictionary of Slang

solid n.

[someone or something that has substance, thus dependable, trustworthy]

1. [late 18C] constr. with the, a guinea.

2. [20C+] (UK tramp) the road.

3. [1920s] (US Und.) a trustworthy fellow criminal or prison inmate.

4. [1950s+] (US black) a favour.

In phrases

do (someone) a/the solid (v.)

[1920s+] (US) to perform a great favour.