Green’s Dictionary of Slang

1. simple, stupid.

1930

R. McAlmon ‘Blithe Insecurities’ in Knoll McAlmon and the Lost Generation (1976) 40: Pop knew Dinkie was only half there.

2. (Aus.) tipsy, semi-drunk.

1955

N. Pulliam I Travelled a Lonely Land (1957) 233/2: half rinsed – ‘half there’.