Green’s Dictionary of Slang

1. unpleasant, rotten, mean.

1947

N. Marsh Final Curtain (1958) 191: All would have been well if Carol Able [...] hadn’t made out a water-tight alibi for that septic child.

2. (Irish) affected.

1996		(con. 1970) G. Moxley Danti-Dan in McGuinness Dazzling Dark (1996) I iii: Here’s her nibs. Look at the walk of her. Septic. She thinks she’s it.
1997		Share Slanguage.