The crossword solution for the term 'PREFIX WITH SEPTIC' is ANTI, which is a four-letter word. The prefix 'ANTI-' is derived from the Greek word 'ANTI, ' meaning 'AGAINST' or 'OPPOSITE.' It is commonly used to describe something that is opposed to or counteracts another thing. In this case, the prefix 'ANTI-' is being used to describe something that is opposed to or counteracts septic, which is a term used to describe something that is contaminated with bacteria or other microorganisms.