c.1600, from Latin septicus "of or pertaining to putrefaction," from Greek septikos "characterized by putrefaction," from sepein "make rotten or putrid, cause to rot" (see sepsis). Septic tank is attested from 1902.
septic sep·tic (sěp'tĭk)
Of, relating to, having the nature of, or affected by sepsis.
Causing or producing sepsis; putrefactive.