Green’s Dictionary of Slang

1. a convict serving a nine-year sentence.

1897

‘Price Warung’ Tales of the Old Regime 219: Blake was a ‘niner’.

2. (Aus.) a woman in her ninth month of pregnancy.

1951

D. Stivens Jimmy Brockett 304: I had a belly now like a niner.

3. see nine n. (1)