Green’s Dictionary of Slang

completely, unreservedly.

1887		Sun (NY) 10 Oct. 2/5: Mac’s been tellin’ me that the Dagos are solid for George.
1916		C.J. Dennis ‘The Singing Soldiers’ in Moods of Ginger Mick 61: They’ve bin up agin it solid since we crossed the flamin’ foam.
1918		Birmingham Dly Gaz. 22 Nov. 2/2: Vote solid for Banfield.
1924		Motherwell Times 24 Oct. 7/1: [advert] Vote Solid for Johnstones — the Working Man’s Complete Outfitter.
1935		J. Conroy World to Win 167: They’re on strike. They’re out solid.
1942		Z.N. Hurston ‘Story in Harlem Sl.’ in Novels and Stories (1995) 1001: Skinny in the hips and solid built for speed.
1953		S. Bellow Augie March (1996) 275: You had to get tied up this way – in dutch good and solid.
1960		C. Hooper Brief Authority 225: I’m getting drunk to-night – solid stinking plastered.
1972		D. Jenkins Semi-Tough 17: That’s really no joke that you hear. They can solid fuckin’ dance.
1986		H.B. Gilmour Pretty in Pink 99: If I really have it solid for a girl [...] I ride by her house.