The Collaborative International Dictionary of English v.0.48:

Septic \Sep"tic\, a. [Septi- + -ic.] (Math.)
   Of the seventh degree or order. -- n. (Alg.) A quantic of the
   seventh degree.
   [1913 Webster] Septic

The Collaborative International Dictionary of English v.0.48:

Septic \Sep"tic\, Septical \Sep"tic*al\, a. [L. septicus, Gr.
   ????, fr. ???? to make putrid: cf. F. septique.]
   Having power to promote putrefaction.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Septic \Sep"tic\, n.
   A substance that promotes putrefaction.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "septic":
   bad, bad for, cankered, contaminated, diseased, foul, gangrened,
   gangrenous, harmful, infected, injurious, insalubrious, insanitary,
   morbid, morbific, mortified, noisome, noxious, pathogenic,
   pathological, peccant, pestiferous, poisoned, polluted,
   putrefacient, putrefactive, rotting, saprobic, saprogenic,
   saprogenous, saprophilous, saprophytic, sphacelated, tainted,
   ulcerated, ulcerous, unhealthful, unhealthy, unhygienic,
   unsanitary, unwholesome