From Longman Dictionary of Contemporary Englishseptic tankˌseptic ˈtank noun [countable] Da large container under the ground, for holding human waste from toilets
Examples from the Corpus
septic tank• It was just a septic tank.• This system is used with a septic tank and soakaway if space is available.• A burst septic tank is believed to have triggered the landslide.• Everywhere, it was like an over-flowing septic tank or something rotting. 