Green’s Dictionary of Slang

(orig. US) a general excl. of approbation, implying a firm bond, honesty, excellence etc.

1916		C.J. Dennis ‘The Push’ in Moods of Ginger Mick 41: An’ they’re drillin’ ’ere together, men uv ev’ry creed an’ kind. / It’s Australia! Solid! Dinkum! that ’as left the land be’ind.
1938		Flash! (Wash., D.C.) 21 Feb. 11/1: solid—An expression that eliminates the use of superfluous words to indicate that everything is exceedingly well with the speaker.
1946		Mezzrow & Wolfe Really the Blues 217: Solid man, any stud that’s all right with Pops must really be in there.
1959		A. Anderson ‘Dance of the Infidels’ in Lover Man 153: ‘Skin’ he said. I slapped his palm real light and he said, ‘Solid.’.
1960		‘Lord Buckley’ Hiparama of the Classics 9: And The All Hip Mahatma say, ‘Solid, Pass the Saki!’.
1970		T. Southern Blue Movie (1974) 172: If that’s what it takes to get a performance out of her . . . solid.
1986		Eble Campus Sl. Oct.
1992		D. Burke Street Talk 2 50: ‘I’ll pick you up at 3.00, okay?’ ‘Solid.’.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 40: Tate said, ‘Solid’.
2014		C. Eble (ed.) UNC-CH Campus Sl. Spring 2014 Fall 6: SOLID — good, I agree: X: ‘I’m going to graduate school next year’ Y: ‘That’s solid’; X: ‘We’re going to Franklin St. for dinner’ Y: ‘Solid. I’ll be there’.