not on, adj. — Green’s Dictionary of Slang

not on adj.

[billiards use, of shots that are ‘not on’ the table; ? also milit. use, when a proposed operation was ‘not on’ through unforeseen circumstances]

socially unacceptable, impossible.

1954		N. Bartlett With Australians in Korea 251: General Everest realized that escape and survival as we knew it in World War II was just ‘not on’ in Korea.
1961		H. Livings Stop it, Whoever You Are (1962) Act II: You’ll see for yourself the sort of thing I have to put up with. It’s not on, you know.
1977		G.F. Newman Prisoner’s Tale 49: Lynn recognised right off what the con was doing [...] It wasn’t on, he decided.
1985		B. Humphries Traveller’s Tool 31: Ermine and heavy jewel-encrusted hats are ‘not on’ in egalitarian Australia.
1993		R. Shell Iced 169: The boy’s got me seeing spiders. And that ain’t on!
2000		J.J. Connolly Layer Cake 125: A couple of boys spoke to his mates, told ’em givin evidence was not on.