Source : Webster's Revised Unabridged Dictionary (1913)

Septic \Sep"tic\, a. [Septi- + -ic.] (Math.)
   Of the seventh degree or order. -- n. (Alg.) A quantic of the
   seventh degree.

Septic \Sep"tic\, Septical \Sep"tic*al\, a. [L. septicus, Gr.
   ????, fr. ???? to make putrid: cf. F. septique.]
   Having power to promote putrefaction.

Septic \Sep"tic\, n.
   A substance that promotes putrefaction.

Source : WordNet®

septic
     adj 1: containing or resulting from disease-causing organisms; "a
            septic sore throat"; "a septic environment"; "septic
            sewage" [ant: {antiseptic}]
     2: of or relating to or caused by putrefaction; "the septic
        action occurs at the bottom of the septic tank"