Green’s Dictionary of Slang

1. a general negative, of a person, not in full possession of their faculties; of a situation, less than satisfactorily under control.

1969		Gandalf’s Garden 6 n.d. 11: untogether: mentally confused, having scattered thinking, unable to cope very well.
1972		Harper’s Mag. Oct. 50: Lots of ofays still think we all play boogie woogie and shine shoes, but I’ll tell you, the most prejudiced, jivest, complex, untogether race in the world is blacks.
2000		J.J. Connolly Layer Cake 7: Someone fucked up by being just too fuckin untogether.

2. (US) unstylish, lacking social awareness.

1972

D. Claerbaut Black Jargon in White America 85: untogether adj. [...] not attractive in manner or socially skillful.