Green’s Dictionary of Slang

forty adj.2

also forte, four-oh, four-point-oh, thirty-eight and two, thirty-eight plus two, twice twenty
[? pun on SE forte, a skill ]

(orig. US black) exceptionally pleasing, very fine.

Beale St Washboard Band [song title] Forty and Tight.
[US]Z.N. Hurston Gilded Six-Bits (1995) 989: Can’t he talk Chicago talk? Wuzn’t dat funny whut he said when great big fat ole Ida Armstong come in? He asted me, ‘Who is dat broad wid de forty shake?’ Dat’s a new word [...] Sometimes he don’t say forty he jes’ says thirty-eight and two and dat mean de same thing. Know whut he tole me when Ah wuz payin’ for our ice cream? He say, ‘Ah have to hand it to you, Joe, Dat wife of yours is jes’ thirty-eight and two. Yes-suh, she’s forte!’.
Dan Burley Orig. Hbk of Harlem Jive 86: Back in 1924 [...] [Jive’s] early advocates described something that pleased them, whether a free dinner, a pretty girl, a new suit, or a pocket full of ‘easy’ money as being ‘forte’ or ‘forty’. This term of approval was also known as ‘twice twenty’ or ‘thirty-eight plus two’.
[US]L. Hughes Tambourines to Glory I iii: You know, forty means fine, O.K., great.
[US](con. 1920s–30s) J.O. Killens Youngblood (1956) 440: Youngblood ain’t feeling so forty today, Mr. Roy. He got a terrible headache.
[US](con. 1940s) M. Dibner Admiral (1968) 245: Nobody ever tells Beauty what a public piece she once was because he is such a four-oh Joe. [Ibid.] 249: You’re looking four-oh, Maude.
[US]D. Ponicsan Cinderella Liberty 158: You look like a pile of dog turds [...] and you always were so four-point-oh.
Vancouver Sun sect. E 31 Jan. 53/6: The rumor can it be true? Are you really forty or just thirty-eight plus two?