up in G, phr. — Green’s Dictionary of Slang

up in G phr.

[? play on the musical note G]

1. (US) very best, superlative.

1881		Daily L.A. Herald 13 Aug. 2/3: A prosperous man is ‘up in G’.
1895		Life (NY) 17 Oct. n.p.: [cartoon caption] I ain’t a’givin’ guff; I’m a’givin’ it to yer straight — he was a corker [...] He was way up in G.
1897		E.W. Townsend Chimmie Fadden 3: Say, I knowed you’d be paralyzed when you seed me in dis harness. It’s up in G, ain’t it?

2. with a loud voice.

1905

H. Green Actors’ Boarding House (1906) 170: Aw, have a sense! A guy can’t open his mouth without you flyin’ away up in G!